Desktop is showing when form is closed

[jdy0803]

In the main window of my program, load form and display as modal like following.
Dim frm As New form1
frm.ShowDialog()
When close from form1 by Me.Dispose(), desktop(background) is showing in a flash.
It looks like this.
mainform=>form1=>close form1=>(display desktop)=>mainform
Flickering occurs at (display desktop)
You can see this symptom when you open Windows Explorer on the background.
I don't know why this happens.
Can anybody give me some advice?
Here is my code

Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim frm As New form2
frm.ShowDialog(Me)
End Sub
End Class
Public Class Form2
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Me.Close()
End Sub
End Class