Anatomical variations of the extensor tendon of the hand are common. However, the majority of anomalous variations are asymptomatic throughout a lifetime and are found incidentally during surgery or after trauma of the hand. The index finger has two independent extensor tendons and lower incidence of anomalous variations than other extensor tendons. We experienced a rare muscular variant of extensor indicis proprius (EIP) during a tendon reconstruction for spontaneous rupture of the 3rd and 4th extensor digitorum communis. Tendon reconstruction using EIP was planned preoperatively. However, EIP was absent and anomalous muscle known as extensor indicis brevis, which originated from the capsular ligament of the wrist and inserted into the ulnar side on the 2nd extensor digitorum communis of the extensor hood, was found. We performed tendon reconstruction using an alternative surgical procedure because extensor indicis brevis was not useful. Attention is required during tendon reconstruction because anatomical variation of EIP may affect a surgical procedure.