怎样才能用ShowMessage？

总是出现：[Error]Project1.dpr(15):Undeclared identifier:'ShowMessage'
请问如何才能用ShowMessage？
程序如下：program Project1;{$APPTYPE CONSOLE}uses
  SysUtils;type
  TOneSet = set of (A, B, C);
var
  OneSet: TOneSet;
begin
  OneSet := [A, B];
  if A in OneSet then
    ShowMessage('集合OneSet包含A');
end;
end.