An automatic analyzer for measuring a sample, comprising: a light source for irradiating a reaction vessel storing the sample with light, and a temperature adjustment mechanism for adjusting the temperature of the light source, wherein the light source includes a first LED and a second LED, wherein the light source further includes: a first optical element that allows at least a part of first light emitted from the first LED to pass; and a second optical element that reflects second light emitted from the second LED, wherein the light source is configured such that light emitted from the second LED is reflected on the second optical element to change an optical path, thereby the light emitted from the second LED is multiplexed with light emitted from the first LED on a same optical axis as that of light emitted from the first LED, wherein the temperature adjustment mechanism is configured by a member that is in contact with the first LED and the second LED respectively, wherein a light intensity of the second LED is controlled to be larger than a light intensity of the first LED, wherein the first optical element is configured to reflect at least a part of the second light, wherein the first optical element and the second optical element are arranged such that the first light having passed through the first optical element and the second light reflected by the first optical element are multiplexed on a same optical axis to become multiplexed light, characterized in that the first LED is a white LED that generates white light in a wavelength range of 350 nm to 800 nm, wherein the second LED is an ultraviolet LED that generates ultraviolet light having a central wavelength of 350 nm or less, further comprising a diffusion member for diffusing the second light, wherein the diffusion member is configured by a diffusion plate that is arranged between the second LED and the second optical element and that diffuses the second light, wherein the first light is diffused within a first diffusion range on a light receiving surface of a light receiver that receives the multiplexed light, wherein the second light is diffused within a second diffusion range on the light receiving surface, and wherein the diffusion member diffuses the second light such that the first diffusion range is included in the second diffusion range. a light source for irradiating a reaction vessel storing the sample with light, and a temperature adjustment mechanism for adjusting the temperature of the light source, wherein the light source includes a first LED and a second LED, wherein the light source further includes: a first optical element that allows at least a part of first light emitted from the first LED to pass; and a second optical element that reflects second light emitted from the second LED, a first optical element that allows at least a part of first light emitted from the first LED to pass; and a second optical element that reflects second light emitted from the second LED, wherein the light source is configured such that light emitted from the second LED is reflected on the second optical element to change an optical path, thereby the light emitted from the second LED is multiplexed with light emitted from the first LED on a same optical axis as that of light emitted from the first LED, wherein the temperature adjustment mechanism is configured by a member that is in contact with the first LED and the second LED respectively, wherein a light intensity of the second LED is controlled to be larger than a light intensity of the first LED, wherein the first optical element is configured to reflect at least a part of the second light, wherein the first optical element and the second optical element are arranged such that the first light having passed through the first optical element and the second light reflected by the first optical element are multiplexed on a same optical axis to become multiplexed light, characterized in that the first LED is a white LED that generates white light in a wavelength range of 350 nm to 800 nm, wherein the second LED is an ultraviolet LED that generates ultraviolet light having a central wavelength of 350 nm or less, further comprising a diffusion member for diffusing the second light, wherein the diffusion member is configured by a diffusion plate that is arranged between the second LED and the second optical element and that diffuses the second light, wherein the first light is diffused within a first diffusion range on a light receiving surface of a light receiver that receives the multiplexed light, wherein the second light is diffused within a second diffusion range on the light receiving surface, and wherein the diffusion member diffuses the second light such that the first diffusion range is included in the second diffusion range.