The novel heterocyclic compound of the present invention is a novel heterocyclic compound having the formula (I) ##STR00001## Y: ##STR00002## R.sup.15--C(R.sup.14).dbd.N--O-- wherein R.sup.1 is H or C.sub.1-6 alkyl, R.sup.2 is H, --CO--C(R.sup.4).dbd.C(R.sup.4)--R.sup.5 wherein R.sup.4 is H or C.sub.1-4 alkyl, and R.sup.5 is C.sub.4-8 alkyl, C.sub.2-8 alkenyl and the like, and the like, Y is the following group wherein X is O or S, R.sup.7 is the same as R.sup.4, R.sup.8 is R.sup.10--C(R.sup.9).dbd.C(R.sup.9)-- wherein R.sup.9 is the same as R.sup.4, R.sup.10 is C.sub.3-6 alkyl and the like, and the like, R.sup.14 is the same as R.sup.4, and R.sup.15 is aryl and the like, Y--(CH.sub.2)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.