PROBLEM TO BE SOLVED: To provide a biofunction observation device which can confirm whether or not radiation irradiation is correctly performed, and a radiotherapy system which can perform correct radiation irradiation.SOLUTION: A radiotherapy system 1 comprises a detection part 11a, a processing part 12, detection parts 21a, 21b, a processing part 22, a function observation part 30 and the like. The detection part 11a and the processing part 12 detect a prompt gamma ray &gammagenerated during a prescribed period (irradiation ray irradiation period) to acquire a first image expressing the generation distribution of positron emission nuclei. The detection parts 21a, 21b and the processing part 22 detect an annihilation gamma ray &gammagenerated due to pair annihilation between the positron emitted from the positron emission nucleus after the prescribed period and the electron to acquire a second image expressing the existence distribution of the positron emission nuclei. The function observation part 30 obtains the residual state of the positron emission nuclei on the basis of the change in the second image with the first image as a reference to obtain the function of a living body 90.
