A vehicle-mounted heat utilization device (10) includes: an engine fluid circuit (12) in which fluid circulates to cool an engine (18); a heater fluid circuit (14) that passes through an exhaust heat recovery apparatus (26) that recovers a heat of exhaust gas (28) and a vehicle-interior heat exchanger (24) for radiating a heat of the fluid into a vehicle interior; a cooling-heating fluid circuit (16) that passes through a radiating heat exchanger (34), the radiating heat exchanger being included in a refrigeration cycle (30) in which refrigerant is compressed by a compressor (32) and the compressed refrigerant is expanded; a first switching unit (52) that is configured to selectively switch a state of the engine fluid circuit (12) and the heater fluid circuit (14) between a connected and a disconnected state; a second switching unit (54) that is configured to selectively switch a state of the heater fluid circuit (14) and the cooling-heating fluid circuit (16) between a connected state and a disconnected state.