A device for radiating pulsed light toward a thrombus in a blood vessel includes a light source configured to output monitoring light to be radiated into the blood vessel, a light detector configured to detect returned light of the monitoring light and output a detection signal, and a computer configured to acquire a time waveform, which is a change in an intensity of the returned light over time, based on the detection signal, wherein the computer is configured to obtain a parameter on the basis of the time waveform and evaluates a reaction in the blood vessel according to the radiation of the pulsed light on the basis of the parameter.