A mobile communication system 100 includes a selection unit 25 configured to select a transition target frequency from the plurality of frequencies when a condition under which a mobile communication terminal not utilize a transition source frequency being any of the plurality of frequencies is fulfilled, and an instruction unit 22 configured to instruct the mobile communication terminal to perform a transition to the transition target frequency selected by the selection unit. The selection unit 25 excludes the transition source frequency from a frequency candidate to be utilized by the mobile communication terminal until an elapsed time after the instruction of the transition to the transition target frequency exceeds a predetermined time.