To prevent irradiation stopping caused by an MLC following error during treatment by controlling a limitation on the amount of the MLC transferred, using such as a conformation therapy method that transfers the MLC while irradiation is being conducted in a radiation therapy.

For multiple leaf openings generated by an MLC opening computation section, a radiation medical treatment planning device controls a part that exceeds a speed limitation amount inputted by a transfer speed limitation amount input section or a speed limitation amount set by a transfer speed limitation amount setting section to make it below the limitation amount by a transfer amount control section or displays it by a transfer amount display section. In another way, it calculates MLC transfer acceleration by a transfer acceleration calculation section and controls a part that exceeds the calculated limitation acceleration amount to make it below the limitation acceleration amount by the transfer amount control section or displays it by the transfer amount display section.