To easily fit a hearing aid to a user and to attain an effective use of the hearing aid by using a characteristic adjuster which can continuously vary the frequency characteristic from almost frequency-proportional monotonous high band amplification through almost inversely frequency proportional monotonous low band amplification around a specific lever by means of a signal variable element.

A characteristic adjuster is added to a hearing aid to continuously adjust the frequency characteristic from almost frequency-proportional monotonous high band increase through almost inversely frequency-proportional monotonous low band increase around 1kHz and without changing the level of this point by means of a single variable element. When the variable part of a variable resistor is set at the right end of this circuit, a capacitor of 0.033μF reaches the connection point between 520kΩ and 47kΩ to show a small effect of a filter and to primarily function as an HPF of 10kΩ and 0.0015μF. Thus, a high band is amplified almost proportional to the frequency. When the variable part of the variable resistor is set at the left end of the circuit, the capacitor of 0.3μF constitutes an LPF against 10kΩ and accordingly a low band is amplified almost inversely proportionally to the frequency.