To provide an agricultural greenhouse heater utilizing the fermentation heat of an organic substance, capable of achieving sufficient heating capability even in a relatively small-scale device by efficiently utilizing the fermentation heat generated inside a tank.

The agricultural greenhouse heater is for heating the interior of an agricultural greenhouse (9) by means of heat exchange by feeding a heating medium heated with the fermentation heat of a substance to be fermented to the agricultural greenhouse. The agricultural greenhouse heater includes a fermentation tank (1) for fermenting the substance to be fermented, and a heating medium circulating device (2) for circulating the heating medium. The heating medium circulating device (2) includes: heat-exchange piping (20) disposed within the fermentation tank (1) for the heat exchange via the heating medium inside the object to be fermented; circulation piping (25) for feeding the heating medium heat-exchanged in the fermentation tank (1) into the agricultural greenhouse (9) for heat exchange and circulating the heating medium between the agricultural greenhouse (9) and the fermentation tank (1); and a blower (26) for making the heating medium flow in the heat-exchange piping (20) and the circulation piping (25).