In an optical/electrical hybrid substrate 10 including a wiring board 11 having a wiring and a via, and an optical waveguide 12 including an optical waveguide body 80 having a first clad layer 81, a second clad layer 83 and a core portion 82 disposed between the first clad layer 81 and the second clad layer 83 and provided on the wiring board 11, and a pair of mirrors 88 and 89 for reflecting a light signal, a first wiring pattern 96 for electrically connecting a terminal 117 of a light emitting device 13 for irradiating the light signal to the wiring and via, and a second wiring pattern 97 for electrically connecting a terminal 119 of a light receiving device 14 for receiving the light signal to the wiring and via, the first and second wiring patterns 96 and 97 are disposed in the optical waveguide body 80.