The optical branching-coupling device having a self-written optical waveguide core is formed without using half mirrors. In the optical branching-coupling device, three POFs are inserted into a housing having an approximately D-shaped sidewall. An approximately semi-columnar region V in the housing was filled with an uncured liquid light-curing acrylic resin. A laser beam was introduced from one of the POFs, and a cured material was formed of the end face of the POF. The diameter was equal to the core diameter of the POF. The cured material grew, resulting in the cured material reaching the end face of another one of the POFs, thereby forming an optical waveguide core. Next, a laser beam was introduced from the end face of the last one of the POFs. The cured material grew, resulting in a connection with the optical waveguide core, thereby forming the optical waveguide core.