In a preamplifier (amplifier) for the radiation detector, an interconnection layer connected to the bonding pad forms one electrode of a feedback capacitor. Since there is no wiring for connecting the bonding pad and capacitor, a parasitic capacitance caused by the wiring will not be generated. Moreover, the capacitor is arranged below the bonding pad with a conductive layer serving as the other electrode, so that the feedback capacitance of the capacitor is included in the parasitic capacitance between the interconnection layer and the substrate. Compared to the conventional case, an amount of capacitance corresponding to the parasitic capacitance caused by wiring and the feedback capacitance for the capacitor is reduced from the input capacitance. Thus, the input capacitance for the amplifying circuit is reduced.