A heat treatment method, comprising: a first step of mist cooling a treatment object retained at a prescribed temperature by supplying mist-like coolant capable of cooling the treatment object by latent heat of vaporization of the coolant from nozzles, to a target temperature (Ta) set according to the treatment object and near to and higher than a martensite transformation point (Ms) at which the structure of the treatment object begins to be transformed into a martensite structure and the target temperature being set between the martensite transformation point and a pearlite transformation point with a temperature higher than the martensite transformation point at which the structure of the treatment object is transformed into a pearlite structure other than the martensitic structure; a cooling slowdown step, following the first step, of supplying the mist-like coolant capable of cooling the treatment object by latent heat of vaporization of the coolant from the nozzles such that the treatment object is mist cooled at a mist density that is less than the mist density of the first step and such that the temperature according to the results of temperature measurement of the outer surface of the treatment object does not reach the martensite transformation point, a second step, following the cooling slowdown step, of retaining the treatment object for a prescribed time in a state where supply of the mist-like coolant is stopped and where the temperatures according to the results of temperature measurement of the outer surface and of the interior of the treatment object are higher than the martensite transformation point (Ms) such that the temperature according to the results of the temperature measurement of the treatment object is rendered approximately uniform at the target temperature; and a third step, following the second step, of cooling the treatment object from a temperature higher than the martensite transformation point (Ms) to a temperature lower than or equal to the martensite transformation point (Ms); wherein the temperature of the treatment object at the time the third step is started is higher than the martensite transformation point (Ms), and in the second step and the cooling slowdown step, temperatures of an outer surface and of an interior of the treatment object are maintained to be higher than the martensite transformation point (Ms) and to be lower than the pearlite transformation point (Ps). a first step of mist cooling a treatment object retained at a prescribed temperature by supplying mist-like coolant capable of cooling the treatment object by latent heat of vaporization of the coolant from nozzles, to a target temperature (Ta) set according to the treatment object and near to and higher than a martensite transformation point (Ms) at which the structure of the treatment object begins to be transformed into a martensite structure and the target temperature being set between the martensite transformation point and a pearlite transformation point with a temperature higher than the martensite transformation point at which the structure of the treatment object is transformed into a pearlite structure other than the martensitic structure; a cooling slowdown step, following the first step, of supplying the mist-like coolant capable of cooling the treatment object by latent heat of vaporization of the coolant from the nozzles such that the treatment object is mist cooled at a mist density that is less than the mist density of the first step and such that the temperature according to the results of temperature measurement of the outer surface of the treatment object does not reach the martensite transformation point, a second step, following the cooling slowdown step, of retaining the treatment object for a prescribed time in a state where supply of the mist-like coolant is stopped and where the temperatures according to the results of temperature measurement of the outer surface and of the interior of the treatment object are higher than the martensite transformation point (Ms) such that the temperature according to the results of the temperature measurement of the treatment object is rendered approximately uniform at the target temperature; and a third step, following the second step, of cooling the treatment object from a temperature higher than the martensite transformation point (Ms) to a temperature lower than or equal to the martensite transformation point (Ms); wherein the temperature of the treatment object at the time the third step is started is higher than the martensite transformation point (Ms), and in the second step and the cooling slowdown step, temperatures of an outer surface and of an interior of the treatment object are maintained to be higher than the martensite transformation point (Ms) and to be lower than the pearlite transformation point (Ps).