PURPOSE: To access aperiodically generated data at random in a free time which is generated specifically by providing a time slot monitor, detector which monitors its output and detects the in-use mode of transmission timing, and access control circuit which sends out an aperiodic signal burst.

CONSTITUTION: The time slot monitor 60 discriminates whether a signal is present or not by the detector 600 and the discrimination value is inputted to a shift register 601. A transmission timing detector 61 detects the transmission timing of time slots by a reference signal generator 6100, switch 6101 which allows the output of a time slot sweeper 602 to pass only when the output of the generator 6100 is "1", smoothing circuit 6120, and discriminator 6103 which discriminates the output of smoothing circuit. Then, whether a transmission period is periodic or not used is known from each output at a terminal 101. An access control circuit 70 stores transmission data in a buffer memory 701 temporarily from an input terminal 102 and then reads and sends it to a modulator according to the output of a transmission timing oscillator 702 which generates a transmission timing signal corresponding to idle timing by the detector 61.