A fermentation state monitoring apparatus includes: a terahertz wave generation element that outputs inspection light using a terahertz wave to a fermented food under fermentation in a sealed product container; a terahertz wave detection element that detects return light of the inspection light reflected by the fermented food in the product container; and a determination unit that determines a fermentation progress of the fermented food based on an index value including a reflectance of the return light with respect to the inspection light or an absorption coefficient of the return light with respect to the inspection light.