Code For Finding Factorials - Overflow Exception ?

I have the following code for finding factorials:

Private Shared Function Factorial(ByVal Number As Long) As Long
If Number = 0 Then
Return 1
Else
Return Number * Factorial(Number - 1)
End If
End Function

It usually results in an overflow. It only works if I start with something small like 4. I have to work with starting numbers such as 30-60. I thought changing the value type to LONG would prevent this problem. This is VB.net just for reference.