A first fluid system includes a thermostat portion T, a valve portion V, and an ECU 30A. The thermostat portion T includes a first thermostat 18 in a first divergent pathway PB1 and a second thermostat 19 in a second divergent pathway PB2. The valve portion V includes a valve mechanism in at least a second portion SG2 of portions SG1, SG2, SG3. In the ECU 30A, achieved is a control unit that controls the valve portion V to switch a flow control state of at least one of the valve mechanism of the valve portion V when one of an open failure and a close failure occurs in one of the thermostats 18, 19.