A valve (41) is capable of regulating the flow rate of cooling water flowing through a main flow passage (Rm). A detour passage (Pd) detours the valve (41) and connects to the main flow passage (Rm) side from the engine (2) side. A fail-safe valve (50) has: a valve body (51) operating independently of the valve (41) and capable of opening and closing the detour passage (Pd); and a temperature detection medium (52) for operating the valve body (51) on the basis of the temperature of cooling water to enable the detour passage (Pd) to be opened and closed. If the portion where an inflow opening (Oin), into which cooling water flowing from an engine (2) flows, branches into a bypass flow passage (Rb) and the detour passage (Pd) is defined as a diverging section (Div), the temperature detectionmedium (52) is entirely contained in the detour passage (Pd) without protruding into the diverging section (Div).