To provide a vacuum generating mechanism in a processing machine for reducing the consumption of cooled water without using heat exchanger.

The vacuum generating mechanism in a processing machine used for generating suction force to a chuck table for holding a workpiece includes a water seal type vacuum pump having a suction hole, a drain hole, and a water feed hole; a suction path passed from the suction hole to the chuck table; a drain tank for storing the drain water discharged from the drain hole; a circulation path passed from the discharge tank to the water feed hole; a water feeding path passed to the water feed hole; a cool water flow rate adjustment means located at the water feed path for adjusting the flow rate of the cool water; a drain water temperature detecting means for detecting the temperature of the drain water stored in the drain tank; and a controller. When the temperature of the drain water stored in the drain tank reaches over a prescribed level, the controller causes the cool water flow rate adjustment means to increase the flow rate.