A check valve (on-off valve device for fluid)(1A) having a valve body assembly (valve body)(3A) that is separated from and made to be in contact with a valve seat in order to open and close a fluid path, where the valve body assembly (3A) has, in order to achieve good sealing properties in both states where a pressure difference is high and where it is low, a valve body (10), a low-pressure seal section (12) formed from a soft material, and a high-pressure seal section (13) formed from a material harder than that of the low-pressure seal section (12). When the fluid path is closed, if the pressure difference between an upstream side path (7) and a downstream side path (8) is relatively low, the low-pressure seal section (12) is in intimate contact with a projection (6) to close the fluid path, and if the pressure difference is relatively high, the low-pressure seal section (12) is compressed and deformed by the pressure difference and, at the same time, the high-pressure seal section (13) is in intimate contact with a valve seat body (4) to close the flow path.