An amplifier and a radiation detector are provided, in which an input capacitance for an amplifying circuit is reduced by improving the arrangement of components. In a preamplifier (amplifier) 1 for the radiation detector, an interconnection layer 51 connected to the bonding pad 42 forms one electrode of a feedback capacitor 13. Since there is no wiring for connecting the bonding pad 42 and capacitor 13, a parasitic capacitance caused by the wiring will not be generated. Moreover, the capacitor 13 is arranged below the bonding pad 42 with a conductive layer 52 serving as the other electrode, so that the feedback capacitance of the capacitor 13 is included in the parasitic capacitance between the interconnection layer 51 and the substrate 61. Compared to the conventional case, an amount of capacitance corresponding to the parasitic capacitance caused by wiring and the feedback capacitance for the capacitor 13 is reduced from the input capacitance. Thus, the input capacitance for the amplifying circuit is reduced.