In an exhaust cleaning system including a DPF in exhaust passages, a pair of temperature sensors is disposed upstream and downstream the filter to detect upstream and downstream exhaust temperatures. An electronic control unit estimates a downstream exhaust temperature from the actual upstream exhaust temperature when an engine is in a predetermined condition such as a cold-start condition or a high speed and high load condition, in which no catalytic reaction heat is generated in the DPF. The control unit then compares the estimated downstream temperature with the actually detected downstream temperature, and determines an exhaust temperature senor malfunction if the difference between the compared temperatures are large.