The object of the present invention is to provide a dosimeter that is small, has a low degree of invasiveness, and includes a sensor portion in response to the shapes of organs, which are complicated and vary between individual organs, and changes in the shapes over time, and a radiotherapy system. The invention is directed to a dose measurement device including: a radiation sensor constituted by a light emitting portion that is made of a polycrystalline scintillator and emits light of intensity dependent on an amount of incident radiation and a cover covering the light emitting portion； an optical fiber that is connected to the radiation sensor and transmits the photons emitted by the polycrystalline scintillator； a photoelectric converter for converting the photons transmitted by the optical fiber into electrical signals； a calculation device for measuring each of the electrical signals through the conversion by the photoelectric converter of each photon, calculating a count rate, and specifying a dose rate； and a display device for displaying measurement results calculated by the calculation device.
