A compound represented by formula (1), or a pharmaceutically acceptable salt thereof is effective as a therapeutic agent or a prophylactic agent for a corneal disease or sensory nerve damage due to corneal surgery, and as a regeneration accelerator for corneal sensory nerves:wherein R1 represents a hydrogen atom or a carboxyl group, R2 represents a hydrogen atom or a hydroxy group, R3 represents a hydrogen atom or a carboxyl group, and R4 represents a hydrogen atom or a hydroxy group.
