如何解决“溢出”

以下程序是求N!的小程序，但在我的机子上只能计算12!，13!就出现“实时错误‘6’溢出”请问还能算更长的数吗？它与什么有关？谢谢!
   Option Explicit
Private Sub sub_jc(ByVal x As Integer, ByRef y As Double)
Dim i As Integer
Dim cj As Long
cj = 1
If x > 0 Then
   For i = 1 To x
      cj = cj * i
   Next i
End If
y = cj
End Sub
Private Sub cmdjc_Click()
Dim inta As Integer
Dim dblb As Double
inta = txtsj.Text
sub_jc inta, dblb
txtjcj.Text = dblb
End Sub