Myxofibrosarcoma is a rare, soft-tissue sarcoma which is classified into three main subtypes according to cellularity and cell atypia: low, intermediate and high grade. A 44-year-old Japanese female presented with a low-grade myxofibrosarcoma on the left forearm. The lesion was completely resected. Histopathologically, the tumor was remarkably hypocellular and exhibited only mild atypia. However, it rapidly invaded into the underlying brachioradial muscle. The present case clearly indicated that a low-grade myxofibrosarcoma can invade into the deeper tissues including muscle even though its histological grade is very low.