Temperature of a sensor chip itself rises with the supply of power owing to temperature dependence of the sensor chip as its basic characteristics. When a chemiluminescence reagent is added at the point of time at which the temperature rise reaches a steady state and a sensor chip photodiode dark current becomes constant, a drastic shift occurs in the sensor chip temperature. Remarkable dispersion occurs at this time in the sensor chip photodiode dark current Variance (unstability) of the sensor chip photodiode dark current can be decreased by reducing the temperature fluctuation of the sensor chip to minimum by using an exothermal effect of a thermal diffusion medium.