A phosphodiesterase 10A inhibitor serving as an effective prophylactic or therapeutic agent for Parkinson's disease, Huntington's disease, Alzheimer's disease, and schizophrenia.The PDE10A inhibitor contains as an active ingredient a pyrazolo[1,5-a]pyridine derivative represented by the following general formula: wherein R1 and R2 are each independently a hydrogen atom or a lower alkyl group having 1 to 4 carbons; and R3 is a hydrogen atom, a lower alkyl group having 1 to 4 carbons, or a lower alkoxyl group having 1 to 3 carbons.