PURPOSE: To perform minute division of attenuation accurately and to use fixed resistors, by inserting an attenuation circuit which selects the attenuation with an AND output of a plurality of pulse signals, between buffers.

CONSTITUTION: When a code signal of 1000 is outputted from a CPU22, output terminals Q1∼Q4 of a latch circuit 23 obtain 1000, transistors (TR) TR1, TR5 are conductive, a resistor 32 is inserted between the output terminal of a resistor 20 of a signal line system and ground, and the attenuation indicated with the ratio of resistors 20 and 32 is obtained. As a result, when an input signal is applied to an input terminal A of a buffer 18, the signal suquentially passes through buffers 18, 19 and 21, is attenuated at a prescribed level and appears at an output terminal B. When a code signal of 1100 is outputted from the CPU22, TR1, TR5 and TR6 are conductive, while an output of an AND circuit 36 goes to 1 and TR9 and TR10 are conductive. Further, when 1111 is outputted from the CPU22, TR1∼TR12 are conductive and the attenuation of the line system is maximized.