A vacuum pressure proportional control valve placed on a pipe connecting between a reaction vessel and a vacuum pump to control the vacuum pressure in the reaction vessel includes a cylinder provided with a piston chamber, a piston housed in the piston chamber so as to make reciprocal linear movement, a valve seat surface, and a valve element which contacts with or separates from the valve seat surface according to the movement of the piston. A stopper member is provided in the cylinder so that a leading end portion of the stopper member is placed in the piston chamber. The stopper member is configured to move the piston back and forth in a moving direction of the piston by use of an adjusting unit to thereby adjust the position of the leading end portion.