OK, I have read existing answers about this question. But no one is tolerable.
See how-to-copy-qt-runtime-dlls-to-project-output
But $$QMAKE_LIBDIR_QT doesn't contain any dll files. All dlls are located in the "bin" directory in qt installation. On the other hand, QMAKE_LIBDIR_QT refers to "lib" directory. So, this method doesn't work.
Do anybody have a working method of doing this thing?