A mobile communication system comprising: a plurality of mobile units; and a plurality of base station units, the mobile communication system being characterized in that, one base station unit of the plurality of base station units sets a length of a period during which at least one mobile unit of the plurality of mobile units monitors another base station unit of the plurality of base station units, the length of the period set by the one base station unit being made depending on a radio access technology monitored by the at least one mobile unit, and depending on a predetermined relationship, the at least one mobile unit stores the predetermined relationship considered by the one base station unit and also sets a length of a period, the length of the period set by the at least one mobile unit being the same as the length of the period set by the one base station unit, the length of the period set by the at least one mobile unit being made depending on the radio access technology, and depending on the predetermined relationship, the length of the period being set independently by each of the one base station unit and the at least one mobile unit, and the predetermined relationship indicates that the length of the period is selected from a plurality of candidate lengths of the period, the plurality of candidate lengths including a first length corresponding to a first radio access technology and a second length corresponding to a second radio access technology, wherein the first length is shorter than the second length, the selection being made depending on the radio access technology to be monitored. a plurality of mobile units; and a plurality of base station units, the mobile communication system being characterized in that, one base station unit of the plurality of base station units sets a length of a period during which at least one mobile unit of the plurality of mobile units monitors another base station unit of the plurality of base station units, the length of the period set by the one base station unit being made depending on a radio access technology monitored by the at least one mobile unit, and depending on a predetermined relationship, the at least one mobile unit stores the predetermined relationship considered by the one base station unit and also sets a length of a period, the length of the period set by the at least one mobile unit being the same as the length of the period set by the one base station unit, the length of the period set by the at least one mobile unit being made depending on the radio access technology, and depending on the predetermined relationship, the length of the period being set independently by each of the one base station unit and the at least one mobile unit, and the predetermined relationship indicates that the length of the period is selected from a plurality of candidate lengths of the period, the plurality of candidate lengths including a first length corresponding to a first radio access technology and a second length corresponding to a second radio access technology, wherein the first length is shorter than the second length, the selection being made depending on the radio access technology to be monitored.