I:=1
WHILE NOT I=LENGTH(DBEDIT1.TEXT) THEN
BEGIN
  IF NOT (DBEDIT1.TEXT[I] IN [0..9]) THEN
  BEGIN
    SHOWMESSAGE('輸入值不是數值');
    DBEDIT1.TEXT:='';
    EXIT;
  END;
INC(I);
END;

uses SysUtils    procedure TForm1.Button1Click(Sender: TObject);
var
  R : Double;
begin
  if TryStrToFloat(Edit1.Text,R) then
    ShowMessage('The number is '   FloatToStr(R))
  else
    ShowMessage('Not Number!');
end;