PROBLEM TO BE SOLVED: To improve sealing performance of a valve seat, and to improve the fatigue life of a diaphragm, in a diaphragm valve. SOLUTION: A valve seat arrangement is equipped with a body 10 having an inlet port and an outlet port for an fluid and a diaphragm 28 connected with the body 10 in the axial direction and fastened between an inflow passage and an outflow passage for controlling flow between the inflow passage and the outflow passage. The diaphragm valve device is provided with a first collar 44 surrounding one of fluid openings and extended to the axial direction towards the diaphragm, a second collar 46 spaced apart in a radial direction towards the outside from the first collar to form a recess between them, and an elastic valve seat ring 40 disposed in the recess. A void region is formed outside the elastic valve seat ring 40. COPYRIGHT: (C)2009,JPO&INPIT