A transmission power of a communication apparatus can be appropriately determined in accordance with different communication multiplicities between two or more communication channels. A mobile communication system in which a base station apparatus and a mobile station apparatus communicate with each other by using configuration channels (CCH) or random access channels (RACH) includes a transmission power control part for determining basic data of the transmission power in RACH of the mobile station apparatus in accordance with a communication status in CCH, and a transmission power determining part for determining the transmission power in RACH of the mobile station apparatus by changing the basic data at least in accordance with the communication multiplicity of RACH.