diff --git a/app/controllers/main_controller.rb b/app/controllers/main_controller.rb
--- a/app/controllers/main_controller.rb
+++ b/app/controllers/main_controller.rb
@@ -16,42 +16,26 @@
   end
 
   def list
-    @problems = Problem.find_available_problems
-    @prob_submissions = Array.new
-    @user = User.find(session[:user_id])
-    @problems.each do |p|
-      c, sub = Submission.find_by_user_and_problem(@user.id,p.id)
-      @prob_submissions << [c,sub]
-    end
+    prepare_list_information
   end
 
   def submit
-    submission = Submission.new(params[:submission])
-    submission.user_id = session[:user_id]
-    submission.language_id = 0
-    source = params['file'].read
-    if source.length > 100_000
-      flash[:notice] = 'Error: file too long'
-    elsif (lang = Submission.find_language_in_source(source))==nil
-      flash[:notice] = 'Error: cannot determine language used'
-    elsif ((submission.problem_id==-1) and 
-	   !(problem=Submission.find_problem_in_source(source)))
-      flash[:notice] = 'Error: cannot determine problem submitted'
-    elsif ((submission.problem_id==-1) and
-	   (problem.available == false))
-      flash[:notice] = 'Error: problem is not available'
-    else
-      submission.problem_id = problem.id if submission.problem_id == -1
-      submission.source = source
-      submission.language_id = lang.id
-      submission.submitted_at = Time.new
-      if submission.save == false
+    @submission = Submission.new(params[:submission])
+    @submission.user_id = session[:user_id]
+    @submission.language_id = 0
+    @submission.source = params['file'].read if params['file']!=''
+    @submission.submitted_at = Time.new
+    if @submission.valid?
+      if @submission.save == false
 	flash[:notice] = 'Error saving your submission'
-      elsif Task.create(:submission_id => submission.id) == false
+      elsif Task.create(:submission_id => @submission.id, 
+                        :status => Task::STATUS_INQUEUE) == false
 	flash[:notice] = 'Error adding your submission to task queue'
       end
     end
-    redirect_to :action => 'list'
+
+    prepare_list_information
+    render :action => 'list'
   end
 
   def get_source
@@ -65,4 +49,17 @@
       flash[:notice] = 'Error viewing source'
     end
   end
+
+  protected
+  def prepare_list_information
+    @problems = Problem.find_available_problems
+    @prob_submissions = Array.new
+    @user = User.find(session[:user_id])
+    @problems.each do |p|
+      c, sub = Submission.find_by_user_and_problem(@user.id,p.id)
+      @prob_submissions << [c,sub]
+    end
+  end
+
 end
+
diff --git a/app/models/submission.rb b/app/models/submission.rb
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -4,6 +4,12 @@
   belongs_to :problem
   belongs_to :user
 
+  validates_presence_of :source
+  validates_length_of :source, :maximum => 100_000, :allow_blank => true, :message => 'too long'
+  validates_length_of :source, :minimum => 1, :allow_blank => true, :message => 'too short'
+  validate :must_specify_language
+  validate :must_have_valid_problem
+
   def self.find_by_user_and_problem(user_id, problem_id)
     subcount = count(:conditions => "user_id = #{user_id} AND problem_id = #{problem_id}")
     if subcount != 0
@@ -28,6 +34,9 @@
   end
 
   def self.find_option_in_source(option, source)
+    if source==nil
+      return nil
+    end
     i = 0
     source.each_line do |s|
       if s =~ option
@@ -62,4 +71,25 @@
     end
   end
 
+  # validation codes
+  def must_specify_language
+    return if self.source==nil
+    self.language = Submission.find_language_in_source(self.source)
+    errors.add_to_base("must specify programming language") unless self.language!=nil
+  end
+
+  def must_have_valid_problem
+    return if self.source==nil
+    if self.problem_id!=-1
+      problem = Problem.find(self.problem_id)
+    else
+      problem = Submission.find_problem_in_source(self.source)
+    end
+    if problem==nil
+      errors.add_to_base("must specify problem")
+    elsif !problem.available
+      errors.add_to_base("must specify valid problem")
+    end
+  end
+
 end
diff --git a/app/views/main/list.rhtml b/app/views/main/list.rhtml
--- a/app/views/main/list.rhtml
+++ b/app/views/main/list.rhtml
@@ -2,6 +2,9 @@
 Current time is <%= format_short_time(Time.new) %>.
 
 <div class="submitbox">
+
+<%= error_messages_for 'submission' %>
+
 <% form_tag({:action => 'submit'}, :multipart => true) do %>
 Problem: <%= select 'submission', 'problem_id', 
              	[['Specified in header','-1']] +