Main Application window font changes when I launch QFileDialog for browsing files on linux

Hello ,

we are using qt 4.8.0 version to build application.

On Red Hat Linux5.4, I am using following code for browsing files/save files.

QFileDialog filedialog (parent_widget,"Select File",dir, filter);
filedialog.setFileMode(QFileDialog::AnyFile);
filedialog.setCaption ("Select File");
filedialog.setViewMode (QFileDialog::Detail);
filedialog.exec();

once the filedialog.exec(); executed , it changes the font of main application. i have attached the snapshot for reference.