A sensor that continuously estimates a PM amount in exhaust gas. The sensor includes a filter member 31 including a plurality of cells that trap PM in exhaust gas; electrode members 32, 33 arranged to face each other with the cell interposed therebetween and forming a capacitor, an electric heater 34 that executes, when a predetermined amount of PM has accumulated in the cells, filter regeneration of heating the cells to combust and remove the PM, a storage unit 40 that stores therein a reference reduction amount, which is an electrostatic capacitance reduction amount between the electrode members 32, 33 in a case where the filter regeneration is executed in a state where the PM is not flowing into the filter member 31, and estimation units 42, 43, 44 that estimate a PM amount based on an actual electrostatic capacitance change amount between the electrode members 32, 33 during a regeneration interval period, and estimate the PM amount based on a difference between an actual electrostatic capacitance reduction amount between the electrode members 32, 33 and the reference reduction amount during a filter regeneration period.