PURPOSE: To obtain an amplifier with high gain and a flat gain characteristic for a broad band of one octave or over by providing a series circuit means comprising an inductor and a resistor connecting between ground and at least one of input side circuit means and an output side circuit means, and a distributed constant line means connecting in parallel with the resistor of the series circuit means having a desired electric length to the amplifier device.

CONSTITUTION: The electric length of a distributed constant line 12 connecting in parallel with a resistor 11 is selected to be 1/4 wavelength with respect to a lower side frequency f0 of a desired band having a band width of nearly one octave. As a result, the effect of the distribution constant line 12 is avoided and the effect of the resistor 11 is relatively increased. On the other hand, the impedance of the distributed constant line 12 is short-circuited with respect to a frequency 2f0 being twice the lower frequency f0 and the effect of the resistor 11 is relatively neglected. Thus, a high gain is obtained with less effect of the resistor 11 at the frequency 2f0 by selecting each element so as to attain impedance matching.