PURPOSE: To provide a light irradiating device, which can improve the sensitizing brightness by the irradiation of light without increasing the total quantity of photon due to the irradiation of light within the predetermined time.

CONSTITUTION: This device is formed of a semi-conductor laser 103 for radiating the continuous pulse light, an input device 100 for inputting pulse width and pulse interval of the continuous pulse light, a memory device 101 for storing the pulse width and pulse interval, a control device 102 for radiating the continuous pulse light from the semiconductor laser 103 on the basis of the stored value, a scattering plate 105 for scattering the radiated continuous pulse light, and a power source circuit 104 for supplying the electric energy to this device. Pulse width of the pulse light is set at 10 micro seconds or less, and the time obtained by subtracting the pulse width from the pulse interval is set at 10 micro seconds or more.