An object of the present invention is to provide a charge amplifier which can be operated at low cost so that electric charge generated in a piezoelectric pressure sensor having one end grounded is converted into a voltage signal. In the charge amplifier (1) according to an embodiment of the present invention, a plus side power source input terminal of an operational amplifier (5) is connected to a plus power source (+5 V) while a minus side power source input terminal of the operational amplifier (5) is grounded, so that the operational amplifier (5) is supplied with a single power source. Further, an offset voltage lower than the plus power source voltage but higher than the ground potential is applied to a non-inverted input terminal of the operational amplifier (5). Accordingly, change of pressure in both positive and negative directions can be converted into a voltage signal with the offset voltage as its center though the operational amplifier (5) is driven by a single power source. That is, cost can be suppressed because a double power source is not required.