<script type="text/javascript">
        function OpenSubForm(ret) {
            var strPath = "subForm.aspx"
            var nHeight = 500
            var nWidth = 500
            var feature
            feature = "Height= " + nHeight + ",Width=" + nWidth + ",top=30,Left=30";
            feature += ",dependent=yes,location=no,resizable=yes,scrollbars=yes,status=yes,toolbar=no;";
            window.open(strPath+"?Ret_Form=Form1&Ret_Value="+ret,'subForm',feature).focus();
            return false;
        }
    </script>

private void Page_Load(object sender, System.EventArgs e)
        {
            // ページを初期化するユーザー コードをここに挿入します
            this.Button1.Attributes.Add("onClick","return OpenSubForm('TextBox1');");
        }

        private void Button1_Click(object sender, System.EventArgs e)
        {
            string strScript =string.Empty;
            string strRetForm = String.Empty;
            string strRetValue=String.Empty;
            strRetForm=Request.Params["Ret_Form"];
            strRetValue=Request.Params["Ret_Value"];
            if (strRetForm == string.Empty)
            {
                strRetForm= "document.forms[0]";
            }
            strScript = "<script language=javascript>";
            strScript += "window.opener." + strRetForm;
            strScript += "." + strRetValue + ".value='" + this.TextBox1.Text.Trim() + "';";
            strScript += "window.close();";
            strScript += "</script>";
            Response.Write(strScript);
        }