A system comprising a fine particle sensor and a mounting structure for mounting the sensor to an exhaust pipe of an internal combustion engine, the sensor being suitable for detecting fine particles in the exhaust gas when the sensor is mounted and exhaust gas is emitted from the internal combustion engine and flowing through the exhaust pipe, the sensor comprising: an ion generating unit (72, 20, 41) for generating ions by corona discharge; a charging unit (71, 31a) for charging with some of the ions generated by the ion generating unit (71, 31 a) the fine particles in the exhaust gas when the sensor is mounted and exhaust gas is flowing through the exhaust pipe; an ion trapping unit (70, 10, 31 b) for trapping a remainder of the ions generated by the ion generating unit (71, 31 a), which remain as excess ions without being used for charging of the fine particles in the charging unit (71, 31a), so that when the sensor is mounted and exhaust gas is flowing through the exhaust pipe, 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 insertable inside the exhaust pipe and accommodating therein the charging unit and the ion trapping unit adjacent to each other in an axial arrangement direction, the casing having, formed in a circumferential wall thereof, a gas inlet hole for introducing 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 (45, 35) are arranged in such a manner as to at least partially overlap each other when the fine particle sensor is viewed in the axial arrangement direction, characterized in that the mounting structure is configured to arrange the sensor with respect to the exhaust pipe in such a manner that, when the sensor is mounted and exhaust gas is flowing through the exhaust pipe, the openings of the gas inlet and outlet holes (45, 35) are directed downstream of the flow of exhaust gas in the exhaust pipe. an ion generating unit (72, 20, 41) for generating ions by corona discharge; a charging unit (71, 31a) for charging with some of the ions generated by the ion generating unit (71, 31 a) the fine particles in the exhaust gas when the sensor is mounted and exhaust gas is flowing through the exhaust pipe; an ion trapping unit (70, 10, 31 b) for trapping a remainder of the ions generated by the ion generating unit (71, 31 a), which remain as excess ions without being used for charging of the fine particles in the charging unit (71, 31a), so that when the sensor is mounted and exhaust gas is flowing through the exhaust pipe, 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 insertable inside the exhaust pipe and accommodating therein the charging unit and the ion trapping unit adjacent to each other in an axial arrangement direction, the casing having, formed in a circumferential wall thereof, a gas inlet hole for introducing 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,