PURPOSE: To provide a failure diagnosing device for a cylinder internal pressure sensor with high diagnosis precision through simple constitution.

CONSTITUTION: When the failure of a cylinder internal pressure sensor is diagnosed, it is decided at S1 whether an engine is under a fuel cut. Only when it is under fuel cut, a detecting value from the cylinder internal pressure sensor is read, and an average effective pressure IMEPR shown in a Fig at a given crank angle section is determined by computation at S2-S8. A previously set and stored theoretically determined average effective pressure IMEPth shown in a Fig is determined at S9. After deviation A between the IMEPR and the IMEPth is determined at S9 and S10, it is diagnosed at S9 and S10 based on a comparing result between the deviation A and a set value at S11-S13 whether the trouble of the cylinder internal pressure sensor occurs. Namely, since only when it is under fuel cut, the trouble of the cylinder internal pressure sensor 7 is diagnosed, a possibility of erroneous diagnosis due to a combustion pressure like a conventional type is eliminated. This constitution performs high-precise and easy diagnosis of the trouble of the cylinder internal pressure sensor.