This invention relates to a 4-oxybenzopyran derivative of formula (I) wherein, R1 and R2 represent each independently a hydrogen atom, a C1-6 alkyl group or a phenyl group; R3 represents a hydroxyl group or a C1-6 alkylcarbonyloxy group; R4 represents a hydrogen atom, a C3-6 cycloalkyl group, a C1-6 alkyl group, a C1-6 alkylcarbonyl group, a C1-6 alkylaminocarbonyl group, a di-C1-6 alkylaminocarbonyl group, an aryl group or a heteroaryl group; n represents an integer of 0-4; X represents —C(═O)NR7—, —NR8—, —NHC(═O)NH— or —S(O)2NH—; R5 represents a hydrogen atom or (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), (k), or (l); R6 represents a hydrogen atom, a halogen atom, a nitro group or a cyano group; or a pharmaceutically acceptable salt thereof. And this invention also relates to an antiarrhythmic agent having the prolongation effect of the functional refractory period comprising said compound or a pharmaceutically acceptable salt thereof as an active ingredient.