Piperidine derivatives represented by formula (I) or nontoxic salts thereof (wherein symbols are defined in the description): ##STR00001## Since the compound represented by formula (I) has a PDE4 inhibitory activity, it is useful for preventing and/or treating inflammatory diseases, diabetic diseases, allergic diseases, autoimmune diseases, osteoporosis, bone fracture, obesity, depression, Parkinson's disease, dementia, ischemia-reperfusion injury, leukemia and the like.