In a low-temperature plasma processor, a sample (23) such as semiconductor substrate is processed with a plasma under a cooled state. An electric insulator (12) is interposed between a sample holder (11) and a coolant container (13) for cooling the sample holder, so as to electrically insulate them. In this way, a bias voltage applied to the sample holder and a voltage for generating the plasma can be prevented from leaking, so as to stabilize the process. In addition, the insulator (12) is held in close contact with the sample holder (11) and the cooling container (13) through thermal conductive members (14), whereby the occurrence of non-uniformity during the process of the sample and variation in the qualities of processed samples among sample lots or among processors can be suppressed.