A liquid hammer prevention device has a casing (K0) led to a fluid channel via passage hole (1), an elastic cushion (50) made of foamed material incorporated in the casing (K0) ; and a diaphragm (F) partitioning the inside of the (K0) into an area led to the fluid channel and another area incorporating the elastic cushion (50). The elastic cushion (50) is "Asker C 30 and 85" according to Japanese Industrial Standard S 6050 measured by the level gauge of a durometer "Asker C," and is 0.30 - 0.70 of apparent specific gravity.