A user equipment for a mobile communication system, and method and chip thereof, receives a first parameter and a second parameter broadcasted from the current serving cell, and reselects a target cell to be used as a serving cell from among plural cells operated at different frequencies. The first parameter defines a probability for performing reselection of the target cell, and the second parameter is a timer value to be set in a timer started at a timing at which a quality measurement for neighbor cells ends. Reselecting includes, after receiving the first and second parameter, periodically measuring qualities of neighbor cells at timer defined timings, comparing a value corresponding to IMSI of the user equipment with a value corresponding to the first parameter, and reselecting the target cell from among cells having a quality satisfying predetermined quality criteria, based on results of the quality measurement and the comparison.