To provide a thermostat bath capable of controlling a temperature with high accuracy at either a higher temperature or lower temperature with respect to room temperature.

A flow channel of the air flow channel 22 is constituted so that the heated air is circulated by an air distribution fan 33 to flow from a lower portion to both side portions, from the both side portions to an upper portion, from the upper portion to a back portion, and from the back portion to the lower portion in the air flow channel 22, when the inside of a thermostat chamber 16 is kept at a temperature higher than room temperature, and the cooled air is circulated by the air distribution fan 33 reversely rotated, to flow from the lower portion to the back portion, from the back portion to the upper portion, from the upper portion to the both side portions and from the both side portions to the lower portion in the air flow channel 22, when the inside of the thermostat chamber 16 is kept at a temperature lower than the room temperature.