INCLUDEPATH in QT creator doesn't work

INCLUDEPATH in QT creator doesn't work

I've got a problem with include in QtCreator, in my .pro file I've got :
INCLUDEPATH += "C:\OpenCV\build\include"
and in my cpp : #include
result : Cannot open include file: 'opencv\cv.h': No such file or directory
but if I write this in my cpp:
include "C:\OpenCV\build\include\opencv\cv.h"
it works!!!
Any idea why?
thanks