A communication terminal apparatus, a terrestrial cellular base station and a mobile communication system are provided, which are capable of being shared in a terrestrial cellular mobile communication system and a satellite mobile communication system, preventing an interference between both systems and improving a spectral efficiency in a downlink of the terrestrial cellular mobile communication system, by simple switching control of radio resources. When the communication terminal apparatus is located in an area of the terrestrial cellular mobile communication system, all of time slots allocated to the terrestrial cellular mobile communication system and at least a part of time slots allocated to the satellite mobile communication system are used in a downlink, and all of time slots allocated to the terrestrial cellular mobile communication system are used and time slots allocated to the satellite mobile communication system are not used in an uplink.