A heat treatment device includes: a heat treatment chamber which accommodates an object to be treated; a cooling gas supply unit which supplies a cooling gas into the heat treatment chamber; a cooling gas circulation unit which circulates the cooling gas in the heat treatment chamber; and a gas purge unit which gas-purges, with an inert gas, a portion in which there is a possibility of mixing of the cooling gas supplied into the heat treatment chamber and an oxygen gas, in which the cooling gas supply unit supplies a hydrogen gas into the heat treatment chamber as the cooling gas.