Goshajinkigan (GJG) is a traditional Japanese Kampo medicine used clinically to treat muscle pain in Japan. However, its underlying mechanism remains unclear. Since voltage-gated sodium channel (Nav) 1.4 is involved in skeletal muscle contraction, we investigated the possibility that GJG may affect Nav1.4 currents. By using an electrophysiological technique on skeletal muscle cell line C2C12, we found that GJG suppresses Nav1.4 currents in C2C12 cells. It is suggested that GJG may improve skeletal muscle stiffness or cramps by inhibiting abnormal Nav1.4 excitation. GJG may act as a Nav1.4 blocker and may be useful to treat muscle stiffness and clamps as well as easing the pain.