A pressure relief valve (56) is opened from a time t 1 to a time t 2 for the purpose of reducing the pressure in the gas phase in a gas-liquid separator (16). A WP (44) is driven at a time t 3 and a time t 4 . The time t 3 and time t 4 correspond to timings at which a difference ”T between a boiling temperature T 1 and an actual temperature T 2 becomes equal to or greater than a predetermined temperature T 3 . Since liquid-phase coolant in a catch tank (42) can be fed to a WP (22) by driving the WP (44), the actual temperature T 2 of the liquid-phase coolant immediately upstream of the WP (22) can be lowered. It is thus possible to prevent intense boiling of the liquid-phase coolant immediately upstream of the WP (22).