An optical encoder includes a light source fiber holder holding a light source fiber and a collecting lens therein, and a nut. The light source fiber holder includes an end fitting together with a light source base. An outer circumferential surface of the end includes an inclined surface inclining such that a cross-sectional area increases as the surface approaches the light source fiber; and a projection capable of advancing into a groove of the light source base. The optical encoder includes a light source connector and the light source fiber holder.