To effectively prevent a sleeper from snoring by effectively changing the body posture of the sleeper.

A left position detecting air bag 10L and a right position detecting air bag 10R are arranged at the lower parts of the front sides of a corresponding left body posture changing air bag 9L and right body posture changing air bag 9R for receiving the head portion of the sleeper, respectively. The Airs in the left position detecting air bag 10L and the right position detecting air bag 10R are led to a pressure sensor via conduits 12L, 12R respectively to detect the pressure of the airs in the position detecting air bags 10L, 10R. The position of the sleeper can be detected on the basis of the pressure in the position detecting bags 10L, 10R and air supplying to and air releasing from the pair of body posture changing air bags 9L, 9R can be performed based on the detecting result.