There is provided an automatic analyzer that keeps a temperature of alight emitting element of alight source constant and has high accuracy analysis performance. A light source for an automatic analyzer includes: a substrate including a light source; a temperature adjustment unit configured to adjust a temperature of the light source; a first optical element configured to emit light from the light source to an outside; and a member configured to cover the light source, in which the temperature adjustment unit, the substrate, the member, and the first optical element are arranged in this order, the optical element is assembled so as to be accommodated in the member, and the temperature adjustment unit puts the light source, the member, and the optical element under the same temperature control.