A light module includes a module main body and a flexible substrate. The module main body has a stem, and a high frequency lead and a DC lead. The flexible substrate has a stem contact portion in contact with the lower surface of the stem. A high frequency through-hole and a DC through-hole are formed in the stem contact portion in which the high frequency lead and the DC lead are respectively inserted. The flexible substrate includes plural surface wirings, a ground wiring provided in a first region which contains the high frequency through-hole, but does not contain the DC through-hole on the back surface of the stem contact portion, and an adhesive layer provided in a second region which is a region excluding the first region on the back surface of the stem contact portion.