PURPOSE: To obtain a preamplifier for a semiconductor detector which enables execution of measurement with high resolution and a high counting rate by mutual supplementation of a pulse reset type discharging intermittently a charge accumulated in a feedback capacity and of a drain feedback type for feeding it back to a drain of FET.

CONSTITUTION: An amplifier 3 connected to a semiconductor detector, a feedback capacity 4 connected in parallel to this amplifier 3, a comparator 6 connected to the amplifier 3, and a power source switch 12 switching a plurality of voltages and a current limiting resistor 13 which are connected in series between an output terminal of the comparator 6 and a drain of FET 2 provided in the initial stage of the amplifier, are provided.