Abstract: This invention relates to compounds (I) or pharmaceutically acceptable salts thereof, having a function to inhibit type IV phosphodiesterase (PDE) ##STR1## [X: an oxygen atom or a sulfur atom, R.sup.1 : a lower alkyl group, a cycloalkyl-lower alkyl group or a cycloalkyl group,R.sup.2 : a hydrogen atom, a halogen atom, a lower alkyl group, a halogeno-lower alkyl group, a hydroxy-lower alkyl group, a mercapto-lower alkyl group, a lower alkoxy-lower alkyl group, a lower alkylthio-lower alkyl group, a lower alkanoyloxy-lower alkyl group, a lower alkanoylthio-lower alkyl group, a lower alkanoyl-lower alkyl group, a hydroxyimino-lower alkyl group, a lower alkoxyimino-lower alkyl group, a cycloalkyl group, an aryl group or a lower alkanoyl group,R.sup.3 : a hydrogen atom, a halogen atom or a lower alkyl group,R.sup.4 : a hydrogen atom or a lower alkyl group,R.sup.

Abstract: 1,8-Naphthyridine derivatives represented by the following general formula (I), salts thereof, hydrates thereof and solvates thereof. ##STR1## They have an activity to inhibit type IV phosphodiesterase and are useful as agents for the prevention and treatment of respiratory diseases, inflammatory diseases accompanying organ transplantation, systemic or local arthropathy, proliferative diseases, micturition-related diseases and diseases in which tumor necrosis factor (TNF) and other cytokine (IL-1, IL-6 or the like) are concerned.