To provide a diaphragm valve device advantageous in reducing the size of a driving part.

The valve device includes a driving part 4 for opening/closing a movable body 2 provided with a valve element 21, and a rolling diaphragm 6 which is flexibly deformed in accordance with operation of the movable body 2. At least either one of a body 1 and the movable body 2 has a slope 7 which comes in face contact with the rolling diaphragm 6 in accordance with an opening/closing operation of the movable body 2. In the slope 7, the effective area for receiving pressure in the rolling diaphragm 6 is varied by varying the curvature of the curved part 63 of the rolling diaphragm 6. The slope 7 is constituted so that the following condition is satisfied; if the effective pressure-receiving area of the rolling diaphragm 6 at valve-closing time of the valve element 21 is represented by Ashut, and the effective pressure-receiving area of the rolling diaphragm 6 at valve opening time of the valve element 21 is represented by Aopen, Aopen is smaller than Ashut.