Abstract: A miniatured semiconductor radioactive ray detector which has an energy resolution substantially equivalent to that of a conventional semiconductor radioactive ray detector without use of liquid nitrogen and is simple as well as readily portable is disclosed. A Stirling type electric refrigerator is provided for cooling the semiconductor radioactive ray detecting element to the temperature near to a cryogenic temperature of liquid nitrogen. The first stage amplification circuit of the signal preamplifier for amplifying the detected signal relating to the radioactive ray and the signal line as well as circuit wiring are covered and molded by a fibrous material having gas permeability and electric insulation properties so as to reduce microphonics which are likely to be generated due to the electric refrigerator. The compression pump associated with the electric refrigerator is fixed to the support base by way of the vibration absorbing material which absorbs the vibration caused by the compression pump.