A sensor (10A; 10B) comprising: a porous filter member (31; 61) including a plurality of cells that traps particulate matter in exhaust gas; and at least a pair of electrode members (32, 33; 62, 63) arranged to face each other with the cell being interposed therebetween and forming a capacitor; characterised in that the sensor further comprises: regeneration means (34, 41; 62, 63) configured to execute, when a predetermined amount of particulate matter has accumulated in the cell, a filter regeneration by heating the cell to combust and remove the accumulated particulate matter; storage means configured to store therein a reference reduction amount (Cp_ref), which is an electrostatic capacitance reduction amount between the pair of electrode members in a case where the filter regeneration has been executed by the regeneration means in a state where the exhaust gas including the particulate matter is not flowing into the filter member; and estimation means (42, 43, 44) configured to estimate a particulate matter amount included in the exhaust gas based on an actual electrostatic capacitance change amount between the pair of electrode members during a regeneration interval period from an end of the filter regeneration by the regeneration means to a start of a next filter regeneration, and to estimate the particulate matter amount included in the exhaust gas based on a difference between an actual electrostatic capacitance reduction amount between the pair of electrode members and the reference reduction amount (Cp_ref) during a filter regeneration period by the regeneration means. a porous filter member (31; 61) including a plurality of cells that traps particulate matter in exhaust gas; and at least a pair of electrode members (32, 33; 62, 63) arranged to face each other with the cell being interposed therebetween and forming a capacitor; characterised in that the sensor further comprises: regeneration means (34, 41; 62, 63) configured to execute, when a predetermined amount of particulate matter has accumulated in the cell, a filter regeneration by heating the cell to combust and remove the accumulated particulate matter; storage means configured to store therein a reference reduction amount (Cp_ref), which is an electrostatic capacitance reduction amount between the pair of electrode members in a case where the filter regeneration has been executed by the regeneration means in a state where the exhaust gas including the particulate matter is not flowing into the filter member; and estimation means (42, 43, 44) configured to estimate a particulate matter amount included in the exhaust gas based on an actual electrostatic capacitance change amount between the pair of electrode members during a regeneration interval period from an end of the filter regeneration by the regeneration means to a start of a next filter regeneration, and to estimate the particulate matter amount included in the exhaust gas based on a difference between an actual electrostatic capacitance reduction amount between the pair of electrode members and the reference reduction amount (Cp_ref) during a filter regeneration period by the regeneration means. regeneration means (34, 41; 62, 63) configured to execute, when a predetermined amount of particulate matter has accumulated in the cell, a filter regeneration by heating the cell to combust and remove the accumulated particulate matter; storage means configured to store therein a reference reduction amount (Cp_ref), which is an electrostatic capacitance reduction amount between the pair of electrode members in a case where the filter regeneration has been executed by the regeneration means in a state where the exhaust gas including the particulate matter is not flowing into the filter member; and estimation means (42, 43, 44) configured to estimate a particulate matter amount included in the exhaust gas based on an actual electrostatic capacitance change amount between the pair of electrode members during a regeneration interval period from an end of the filter regeneration by the regeneration means to a start of a next filter regeneration, and to estimate the particulate matter amount included in the exhaust gas based on a difference between an actual electrostatic capacitance reduction amount between the pair of electrode members and the reference reduction amount (Cp_ref) during a filter regeneration period by the regeneration means.