Ultraviolet absorbent, photostabilizer, ultraviolet ray-absorbing composition, photostabilized composition and external prepraration for skin

An ultraviolet absorbent comprising as the effective ingredient the following pyridazine derivative:

• 4
• wherein R1 and R4 independently represent hydrogen atom, hydroxyl group, a lower alkyl group, a lower alkoxyl group or N R5, R6 group, wherein R5 and R6 may be the same or different and represent hydrogen atom, a lower alkyl group, or a lower hydroxyalkyl group or R5 and R6 taken together with nitrogen atom represent a heterocyclic group selected from the group consisting of aziridinyl group, azetidinyl group, pyrrolidinyl group, piperidino group, hexahydroazepinyl group, heptamethylene imino group, octamethylene imino group, morpholino group, thiomorpholino group, piperazinyl group, and 4-lower alkylpiperazinyl group;
• R2 and R3 independently represent hydrogen atom, bromine atom, chlorine atom, hydroxyl group, a lower alkyl group, a lower alkoxyl group, or NR7, R8 group, wherein R7 and R8 may be the same or different and represent hydrogen atom, a lower alkyl group, or a lower hydroxyalkyl group;
• or R7 and R8 taken together with nitrogen atom represent a heterocyclic group selected from the group consisting of aziridinyl group, azetidinyl group, pyrrolidinyl group, piperidino group, hexahydroazepinyl group, heptamethylene imino group, octamethylene imino group, morpholino group, thiomorpholino group, piperazinyl group, and 4-lower alkylpiperazinyl group;
• and wherein R1, R2, R3 and R4 are not simultaneously hydrogen atom and R2 and R3 are not simultaneously morpholino group;
• and a salt thereof.