[Task] There is provided a mold release agent for forming a water glass-containing sand mold with high release properties which can prevent attachment of a water glass to a flask and the accumulation of the water glass on the flask in the formation of the sand mold when using the water glass as a binder.[Solution] The mold release agent for forming the water glass-containing sand mold is structured to contain an organic compound, the organic compound having a proton-donating functional group that reacts with the water glass to produce silicon dioxide. The water glass contained in a raw material for the sand mold reacts with the proton-donating functional group contained in the mold release agent so as to produce silicon dioxide. Since silicon dioxide has no adhesion, it can prevent that the attachment (fixing) of the water glass contained in a formed sand mold to the flask and the accumulation of the water glass on the flask.