A radioactive dust monitor comprising a dust collecting electrode serving also as a radiation detecting surface, an ionization line extending in air in a spaced relation with the dust collecting electrode and a radiation detection part. The pair of oil sumps is formed in a switch-spool insertion hole and enter a neutral state or a state of being selectively communicated with any one of respective pressure-oil paths and respective oil-return paths. In addition, an adjustment capacitor is connected between the reflective electrode and a different common line. The system can include a wand-like arm having a slot through which the photograph is inserted. Therefore, the internal combustion engine is capable of reliably combusting HC, CO, smoke, and so forth remained in a combustion chamber due to a quenching phenomenon or the like.