Thank you very much! That has worked. But now I encountered another problem.
When I'm calling QPrintPreviewDialog, there is two pages, thow I rendered only my central widget. So I have one page with my central widget and another page empty.
Also the Image is printed correctly on the page only if I open the app in fullscreen. If I call my app and it is not stretched to full screen than I see onle part of it on the first page.
I provide the code of my print preview SLOT and connects.
Qt Code: 
  1. connect(prev_dialog, SIGNAL(paintRequested(QPrinter*)), this, SLOT(print_preview()));
  2.     connect(ui->actionPrint_Preview, SIGNAL(triggered(bool)), this, SLOT(preview_called()));
  3. //////////////////////////////////////////////////////////////////////////////////////////
  4. void MainWindow::print_preview(void)
  5. {
  6.     QPainter painter(&printer);
  7.     painter.setWindow(this->geometry());
  8.  
  9.     this->render(&painter);
  10.     printer.newPage();
  11.     /*if (prev_dialog->exec() == QDialog::Accepted)
  12.     {
  13.  
  14.     }*/
  15. }
  16.  
  17. void MainWindow::preview_called(void)
  18. {
  19.     prev_dialog->exec();
  20. }
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Thank you again for helping me to deal with this.