A process for producing a compound represented by formula (1), wherein each of R1, R2, R3 and R4, which may be the same or different, represents a hydrogen atom or a lower alkyl group, the process includes deprotecting a protective group (R5) of a compound represented by formula (2), wherein R1, R2, R3 and R4 are as defined above, and R5 represents a protective group of a hydroxy group. The process is an industrially advantageous, simple, and efficient process for producing a key intermediate of a benzo[b][1,4]diazepine-2,4-dione compound, which is a therapeutic medicine for arrhythmia.