The novel heterocyclic compound of the present invention is a novel heterocyclic compound having the formula (I) Y: R15—C(R14)═N—O— wherein R1 is H or C1-6 alkyl, R2 is H, —CO—C(R4)═C(R4)—R5 wherein R4 is H or C1-4 alkyl, and R5 is C4-8 alkyl, C2-8 alkenyl and the like, and the like, Y is the following group wherein X is O or S, R7 is the same as R4, R8 is R10—C(R9)═C(R9)— wherein R9 is the same as R4, R10 is C3-6 alkyl and the like, and the like, R14 is the same as R4, and R15 is aryl and the like, Y—(CH2)n-O— is bonded to the 6- or 7-position of the tetrahydroisoquinoline skeleton, and n is an integer of 1 to 4, or a pharmaceutically acceptable salt thereof. The compound (I) of the present invention is useful as a hypoglycemic agent, a hypolipidemic agent, an insulin resistance improver, a therapeutic agent of diabetes, a therapeutic agent of diabetic complications, a glucose tolerance improver, an anti-arteriosclerosis agent, an anti-obesity agent, an antiinflammatory agent, an agent for the prophylaxis or treatment of PPAR-mediated diseases or an agent for the prophylaxis or treatment of syndrome X.