A soot detecting apparatus includes: a high voltage generating unit for generating a high voltage; a discharge sensor which includes a pair of electrodes to be, disposed in an atmosphere to be detected, for causing a spark discharge at a discharge voltage dependent upon soot concentration in the atmosphere when the high voltage is applied across the pair of electrodes; a first detection unit for detecting a first discharge voltage value when the spark discharge occurs in the discharge sensor upon application of the high voltage of a predetermined voltage polarity; a first output unit for determining a first soot concentration value based on the first discharge voltage value; and a changeover unit for converting the predetermined voltage polarity to an opposite voltage polarity for causing reverse spark discharge at the pair of electrodes of the discharge sensor.