To prevent a check valve in a main nozzle duct from opening even when vibration occurs by positioning the check valve on an adjusting chamber side of the main nozzle duct and forming the main nozzle duct as a long linear duct which protrudes into a venturi part on a downstream side of the check valve.

A main nozzle duct 20 protrudes into a suction duct 40 in a venturi part 43 region. Moreover, the main nozzle duct 20 is formed as a long and linear duct 22 in a mounting part 30 and is extended from an end part 32 on an end face side of the mounting part 30 to an end part 38 on the other end face side substantially. A check valve 25 opened in the direction of the long and linear duct 22 is arranged in a valve chamber 26. The check valve 25 has a valve plate piece 4 as a valve member, and the valve plate piece 24 can move freely between a valve seat 29 and a support ring 35 in the valve chamber 26. The check valve 25 is positioned in an end part of the long and linear duct 22 enlarged so as to form the valve chamber 26.