Patent ID: 8652240

Claim:
A fine particle sensor for detecting fine particles in exhaust gas flowing through an exhaust pipe of an internal combustion engine, comprising: an ion generating unit that generates ions by corona discharge; a charging unit that charges, with some of the ions generated by the ion generating unit, the fine particles in the exhaust gas emitted from the internal combustion engine through the exhaust pipe; an ion trapping unit that traps a remainder of the ions generated by the ion generating unit, which remain as excess ions without being used for charging of the fine particles in the charging unit, so that the fine particle sensor can generate an output signal responsive to the amount of the fine particles in the exhaust gas according to the amount of the excess ions trapped by the ion trapping unit; and a casing inserted inside the exhaust pipe and accommodating therein the charging unit and the ion trapping unit adjacent to each other in a given arrangement direction, the casing having, formed in a circumferential wall thereof, a gas inlet hole for introducing the exhaust gas from the exhaust pipe into the charging unit and a gas outlet hole for discharging the exhaust gas that includes the fine particles charged with the some of the ions generated by the ion generating unit, out from the ion trapping unit to the exhaust pipe, wherein the gas inlet and outlet holes are arranged in such a manner as to at least partially overlap each other when the fine particle sensor is viewed in the given arrangement direction.