To provide a passive decay heat removal device for nuclear reactor capable of effectively and sufficiently removing decay heat generated after a scrum of a nuclear reactor without needing power equipment or power to actuate the equipment.

The decay heat removal device includes: a gravity type coolant injection water tank 27 provided over the water level of coolant in a reactor pressure vessel 4 for storing a predetermined amount of coolant; a water supply pipe 28 for linking the water tank 27 to the reactor pressure vessel 4; an on-off valve for water supply 29 provided in the middle of the water supply pipe 28 which is opened when the temperature of the coolant is equal to or over a predetermined temperature due to shutdown of the operation of a core cooling system in the case that nuclear fuel 2 generates decay heat through controlling of fission chain reaction by control rods 6; and a pressure equalization pipe 30 for making the pressure of space part of the water tank 27 equal to the internal pressure of the reactor pressure vessel 4.