According to one embodiment, in a case where a DC electric field of 5 kV/mm is applied to a silicon nitride sintered body at 25° C., a ratio Q25(600)/Q25(5) of an accumulated charge amount 600 seconds after a start of application to an accumulated charge amount 5 seconds after the start of the application is not less than 1.0 and not more than 3.0. A thermal conductivity of the silicon nitride sintered body is 65 W/m·K or more.