To reliably exhaust air in hydraulic fluid chamber, and to prevent reduction in capacitive efficiency of a pump and the deterioration of a pulsation rate by arranging an air housing hole in an oil reservoir of an air vent valve device, and arranging here a valve operated by a valve driving mechanism and capable of selectively taking first to third positions.

In an air vent valve device for communicating with hydraulic fluid chamber of a diaphragm pump via a through hole 10, at the beginning, an air vent valve 15 is put in a first position where air in the hydraulic fluid chamber enters an air housing hole 45 of a rotary valve 34 via the through hole 10 and a substituting chamber communicating hole 37 of a fixed valve 32. When oil pressure generated by a valve driving mechanism 27 for synchronizing with rotation of a cam 29 is supplied in a cutout part 46 in the rotary valve 34 through a tube 28 in this state, a movable side valve of the rotary valve 34 rotates, and becomes a second position for cutting off the air housing hole 45 and the substituting chamber communicating hole 37. Next, the air housing hole 45 is switched to a third position for communicating with an oil reserving tank communicating hole by further rotation of the movable sidde valve to exhaust air.