It is intended to disclose a glutamate sensor in taste sensation and digestion and provide techniques using the same. A glutamate-receptor protein having the following characteristics is reacted with a substance binding thereto in the presence of a test substance and thus the inhibition or acceleration of the reaction is detected to thereby search for a glutamate agonist or antagonist or an allosteric modulator: (1) having a transmembrane domain and an intracellular domain common to a type 1 metabolic glutamate receptor protein; and (2) having an extracellular domain shorter by about 409 or 481 amino acid residues than the type 1 metabolic glutamate receptor protein.