A transmission/reception system having a transmitter and a receiver can be used in an encapsulated endoscope which can be swallowed into a biological body. The transmitter has a voltage converter for generating voltage according to transmission data, a voltage control oscillator for generating a signal of frequency according to the voltage generated by the voltage converter by non-feedback control, and a first antenna for emitting the signal generated by the voltage control oscillator. The receiver has a second antenna for receiving the signal emitted from the first antenna of the transmitter, a first amplifier for amplifying the signal received by the second antenna, an oscillator for generating a local oscillation signal, a mixer for mixing the signal amplified by the first amplifier with the local oscillation signal and converting the signal amplified by the first amplifier into an intermediate frequency signal, a detector for detecting the intermediate frequency signal, and a control unit for changing the frequency of the local oscillation signal according to the frequency of the received signal so that the intermediate frequency signal has a predetermined frequency. In this transmission/reception system, the transmitter can have a small size and power consumption can be reduced.