PURPOSE: To obtain a linear amplifier without using any resistor and any adjustment by forming this amplifier with an operational amplifier, a capacitor and a switching means.

CONSTITUTION: An input signal is applied to an inverting input terminal (-) of the operational amplifier 4 from a signal input termianl 1 via the switching means 6 and the capacitor 7. The amplification factor of the operational amplifier 4 is decided by a capacitance value C1 of the capacitor 7 and a capacitance value C2 of a capacitor 8 for feeding back output. The switching means 9 is interlocked with the switching means 6, and when the means 6 is connected to the position A, the means 9 is connected to the position A similarly and the means 6, 9 are designed to be connected to the position B at the same time conversely. When both means 6, 9, are connected to the position B, the charge stored to the inverting input terminal (-) of the operational amplifier 4 because of leakage current or the like is eliminated. When both means 6, 9 are connected to the position B, the substantial amplifying function is obtained.