A submount which has a mounting surface on which three or more semiconductor lasers are arranged in a first direction, and includes a heat generator configured to increase the temperatures of the three or more semiconductor lasers, in which, where the heat generator generates heat, a first heat of the heat absorbed by a first semiconductor laser of the three or more semiconductor lasers disposed at one end along the first direction is larger than a second heat of the heat absorbed by a second semiconductor laser of the three or more semiconductor lasers disposed to be adjacent to the first semiconductor laser on the mounting surface.