To provide a radiotherapy system which is equipped with a radiotherapy planning device (CT simulator) having as base body an X-ray CT device capable of drafting a treatment plan to match an irradiation field of a radiotherapy apparatus just as a radiotherapy planning device (X-ray simulator) having a radiography device as base body.

A radiotherapy planning device 110 includes a scanner 1, a first projector 2, a second projector 3, a photographing base 5, a first communication circuit 7, a first console 8, an isocenter setting means 9, a virtual ray source setting means 10, a data storing means 11 and an arithmetic means 12. The isocenter setting means 9 and the virtual ray source setting means 10 set an isocenter and a virtual ray source based on a tomographic image and a scanogram image and the arithmetic means 12 generates and displays a transmission image based on the tomographic image assigned by the isocenter and the virtual ray source previously set to set an irradiation field on the transmission image. This enables drafting of a treating plan to match the irradiation field of a radiotherapy apparatus just as for the irradiation field settable by an X-ray simulator.