Abstract: An automatic analyzing apparatus which can make an adjustment to a target temperature so as to stabilize the light intensity of an LED and also can make the adjustment to that temperature in a short time is provided. The automatic analyzing apparatus in which an LED is used for a light source 114 includes a temperature adjusting mechanism 201 for the LED, and the temperature adjusting mechanism 201 includes a metal member 202 provided with the light source 114, a pair of metal pipes (water-flowing pipes) 203 buried in the metal member 202 and allowing constant-temperature bath water to flow therein, and pins (small metal piece members) 204 bringing only a heat-generating lead 252 of the light source 114 into direct contact with the metal member 202. Therefore, the temperature adjusting mechanism 201 can make an adjustment to a target temperature so as to stabilize the light intensity of the LED, and the adjustment to that temperature can be made in a short time.