NEW MATERIAL:A phthalazinone derivative shown by formula I (R1 is alkoxy; R2 is H or alkyl; R3 is halogen, hydroxyl group, alkoxy, nitro, amino, acylamino, cyano, carbamoyl, carboxyl, alkoxycarbonyl or diazonium; R4 is hydroxyl group, alkoxy or R3 and R4 are bonded to form O or (O-O-), a salt or a solvate thereof.

EXAMPLE: 7-Ethoxycarbonyl-4-hydroxymethyl-6,8-dimethyl-5-nitro-1(2H)- phthalazinone.

USE: Having excellent inhibitory action on blood platelet aggregation and useful as a medicine for thrombosis, arteriosclerosis, cerebral apoplexy, etc.

PREPARATION: A compound shown by formula II (R'4 is hydroxyl group or alkoxy) is nitrated or a compound shown by formula III is reduced to give a compound shown by formula I.