The present invention provides a method of inputting light into a core layer of a multimode optical waveguide. The optical waveguide includes an under-cladding layer; and the core layer formed so that at least a part of the core layer is adjacent to the under-cladding layer. The method includes inputting the light into the core layer so that a spot of the light at an input port-side end surface of the multimode optical waveguide completely includes an input port of the core layer; a spot area of the light at the input port-side end surface of the multimode optical waveguide is twice or more of an area of the input port of the core layer; and an angle of input range of the light at an input port end surface of the core layer is from 10° to 30°.