An apparatus for measuring absorption dose distribution may be used for radiotherapy, such as intensity modulated radiotherapy and radiosurgery. In the apparatus, measurement or evaluation of the distribution of the radiated dose within a phantom can be achieved accurately and in a relatively short length of time. The apparatus includes a phantom with a plastic plate scintillator having a thickness within the range of 0.15 to 1 mm and plastic blocks sandwiching the plastic scintillator, and an image analyzer. At least one of the plastic blocks is transparent and the image analyzer measures a pattern of intensity distribution of light emitted from the plastic scintillator when the phantom is irradiated.