so that the band of preamplifier 2 becomes constant input impedance is controlled as one means to control the band of preamplifier 2 more specifically this is enabled by altering either a resistance value of a feedback resistor 3 or a gain of a variable gain amplifier 6 0044 fig6 shows one embodiment of detecting the band of preamplifier 2 by means of band detection circuit 1 by which it becomes possible to detect the band directly from an output of preamplifier 2 in other words because it is an object to control the band of preamplifier 2 it is direct and most effective to detect the band from the output of preamplifier 2 also according to the band detection method shown in fig7 an output signal of preamplifier 2 is linearly amplified by an agc automatic gain control circuit 44 to a certain value irrespective of the output amplitude of preamplifier 2 this signal is input to band detection circuit 1 therefore band detection circuit 1 has a signal for detection having sufficient amplitude thus band detection with a small error can be performed 0046 fig8 shows a configuration example of band detection circuit 1 band detection circuit 1 is constituted by a band detector 7 and a detection signal hold circuit 8.