PURPOSE: To obtain the subject compound useful as an intermediate for producing benzimidazolone pigment useful for coloring plastics, etc., by reacting 5- aminobenzimidazolone with diketene in a polar organic solvent in a specific temperature range.

CONSTITUTION: 5-Aminobenzimidazolone is reacted with diketene in a polar neutral organic solvent such as dimethylformamide at -5 to +120°C for 1 hour while stirring by dripping the diketene, the organic solvent is distilled away, the reaction mixture is cooled to room temperature, added to water while stirring, a formed precipitate is filtered, separated, washed with methanol and dried at 70-90°C to give the objective 5-acetoacetylaminobenzimidazolone composed of whilely pink crystal suitable as an intermediate for producing benzimidazolone pigment useful for coloring plastic, gravure ink and coating compound for automobiles in high productivity and in high yield.