PURPOSE: To obtain the amplified reproduction signals of nearly the same characteristics by variably controlling the emitter current of an emitter grounding type transistor amplifier means in accordance with a reading out rate and amplifying the reproduction signals by the amplification gain corresponding to the emitter current.

CONSTITUTION: The collector of a transistor Q1 is connected to the emitter of a transistor Q2. A 3rd current I3 or 3rd and 4th currents I3+I4 are supplied through a 3rd constant current source 6A connected to a power source VCC and a switch circuit 7 and a 2nd constant current source 6B connected in series to the mid-point a of the connection. A prescribed voltage is impressed from a power source 8 to the base of the TRQ2 and the collector is connected through a load resistor R2 to the power source VCC. The gain A of the TRQ1 attains the value expressed by equation I and the collector current obtd. by amplifying the inputted reproduction signal SPB10 with a gain A is generated. As a result, the voltage generated in the load resistor R2 is delivered as the amplified reproduction signal SPB11.