The analyzer according to the embodiment comprises an irradiation optical part that irradiates a mixture inside reaction tubes with light from a light source. Moreover, a detection optical part detects light transmitted through the mixture. Moreover, the irradiation optical part comprises a first optical element in which the light source is disposed at the front focal position and that concentrates light from the light source. Moreover, a second optical element guides light transmitted through the first optical element to the reaction tubes. In addition, an incident numerical aperture adjustment member is provided at the rear side of the first optical element and adjusts the numerical aperture when light from the light source is incident on the reaction tubes.