A signal transmission system for transmitting an UP LINK signal received by an antenna shared by at least a first radio communication system and a second radio communication system to at least a radio base station of the first radio communication system is provided. In the signal transmission system, the first radio communication system is affected by a spurious signal of a radio communication system other than the first radio communication system, and the signal transmission system includes: a determination part for determining whether a signal level of a signal, included in the UP LINK signal, from at least the second radio communication system exceeds a permissible level; and a level adjustment part for adjusting a power level of the UP LINK signal so as to output an adjusted signal according to the determination result, and sending the adjusted signal to the radio base station.