A moderator temperature coefficient measuring device and a moderator temperature coefficient measuring method are provided, which enable measurement of the temperature coefficient of the moderator in a preferred manner. Included are a control rod driving device 17 capable of inserting a control rod 16 into a fuel assembly 15 stored in a nuclear reactor 5; a reactivity measurement processing unit 51 capable of measuring the reactivity of the reactor 5; an inflow temperature sensor 11 capable of sensing the inflow temperature of a reactor coolant flowing into the reactor 5; an outflow temperature sensor 12 capable of sensing the outflow temperature of the reactor coolant flowing out of the reactor 5; an average temperature measurement processing unit 52 capable of measuring the average temperature from the sensed inflow and outflow temperatures; a power control unit 19 for providing control so that the reactor 5 delivers constant thermal power; and a temperature coefficient calculating unit 53 for calculating the temperature coefficient of the reactor coolant based on the amount of change in reactivity between before and after the insertion of the control rod 16 and the amount of change in average temperature between before and after the insertion of the control rod 16 under the constant thermal power.
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