A particulate matter detecting apparatus (S) comprises a sensor body (S1) which has a sensor device (1) which is retained in a housing (H) secured to an exhaust pipe (101) of an internal combustion engine (ENG) and detects particulate matter contained in exhaust gas, a sensor temperature determining unit (2) which works to determine a temperature of the sensor device, and a mounted condition diagnosis unit (3) which diagnoses a mounted condition where the sensor body is mounted in the exhaust pipe. The mounted condition diagnosis unit includes a diagnosis threshold setting unit (31) and a mount error determining unit (32). The diagnosis threshold setting unit determines a diagnosis threshold (Tth), as used in a diagnosis of the mounted condition, as a function of an operating condition of the internal combustion engine to have a temperature value lower than a temperature of the sensor device when the sensor body is normally mounted in the exhaust pipe. When a sensor temperature (T), as determined by the sensor temperature determining unit, is lower than the diagnosis threshold, the mount error determining unit determines that an error in the mounted condition is occurring.