The present invention conducts the initial synchronization acquisition of the rapid and high precision ultra-wideband signal without complicatedness of hardware and increase in power consumption. For this purpose, a communication apparatus for exchanging information with an intermittent pulse train signal searches all phases among the pulses in the predetermined search resolution in the process to acquire initial synchronization of the input pulse, estimates the region where the peak phase of the largest output value exists, narrows the region where the peak phase exists up to the predetermined range by repeating the search for all phases in the estimated region in the next step, and conducts acquisition of detailed synchronization in the estimated region. In every step, the threshold value for judging existence of signal or a gain in the analog circuit is controlled for each step. Moreover, the search resolution is set coarse for estimation of the peak phase and set fine for acquisition of detailed synchronization.