An exhaust gas cleaning system for a diesel engine, comprising: a filter disposed in an exhaust system of the diesel engine in order to trap any particulate matter included in exhaust gases; first and second pressure sensors (21, 22) disposed at upstream and downstream sides, respectively, of said filter; and differential pressure computing means for computing, on the basis of pressures (P1, P2) detected by said first and second pressure sensors (21, 22), a differential pressure existing across said filter, the differential pressure across said filter computed by said differential pressure computing means being used for the system to judge whether said filter needs to be regenerated by burning off the particulate matter deposited therein; characterized in that said exhaust gas cleaning system includes: a first temperature sensor emplaced at the upstream side of said filter; and a second temperature sensor emplaced at the downstream side of said filter, wherein when pressure to be measure are applied from a pressure-introducing compartment to one side of a diaphragm to which is attached a strain gauge to from part of a measuring circuit and an opposite side of the diaphragm is opened for exposure to the atmospheric pressure, the first and second pressure sensors detect the to-be-measured pressures relative to the atmospheric pressure, wherein said exhaust gas cleaning system includes: correction data computing means by which, each time a key switch for starting the diesel engine is turned on and before the diesel engine is started, any differences of the pressures detected by said first and second pressure sensors, with respect to a predetermined reference pressure value (Pref), are computed to obtain first and second correction values (ΔP1, ΔP2); and correction data storage means into which the first and second correction values computed by said correction data computing means are stored, wherein said differential pressure computing means is adapted to use, in addition to the pressure detection results of said first and second pressure sensors, the first and second correction values stored in the correction data storage means, to compute the differential pressure across said filter, and wherein only when a difference between temperatures detected by the first and second temperature sensors is within a defied data range and temperatures detected by the first and second temperature sensors and within respective defined data ranges, does said correction data computing means calculate the correction values (ΔP1, ΔP2) and store the calculated values into the correction data storage means (42. a filter disposed in an exhaust system of the diesel engine in order to trap any particulate matter included in exhaust gases; first and second pressure sensors (21, 22) disposed at upstream and downstream sides, respectively, of said filter; and differential pressure computing means for computing, on the basis of pressures (P1, P2) detected by said first and second pressure sensors (21, 22), a differential pressure existing across said filter, the differential pressure across said filter computed by said differential pressure computing means being used for the system to judge whether said filter needs to be regenerated by burning off the particulate matter deposited therein; a first temperature sensor emplaced at the upstream side of said filter; and a second temperature sensor emplaced at the downstream side of said filter, wherein when pressure to be measure are applied from a pressure-introducing compartment to one side of a diaphragm to which is attached a strain gauge to from part of a measuring circuit and an opposite side of the diaphragm is opened for exposure to the atmospheric pressure, the first and second pressure sensors detect the to-be-measured pressures relative to the atmospheric pressure, correction data computing means by which, each time a key switch for starting the diesel engine is turned on and before the diesel engine is started, any differences of the pressures detected by said first and second pressure sensors, with respect to a predetermined reference pressure value (Pref), are computed to obtain first and second correction values (ΔP1, ΔP2); and correction data storage means into which the first and second correction values computed by said correction data computing means are stored, wherein said differential pressure computing means is adapted to use, in addition to the pressure detection results of said first and second pressure sensors, the first and second correction values stored in the correction data storage means, to compute the differential pressure across said filter, and wherein only when a difference between temperatures detected by the first and second temperature sensors is within a defied data range and temperatures detected by the first and second temperature sensors and within respective defined data ranges, does said correction data computing means calculate the correction values (ΔP1, ΔP2) and store the calculated values into the correction data storage means (42.