To make any state easily diagnosable even in case of an operative diesel engine while simplifying an outer boundary around the engine.

Combustion pressure in a diesel engine 1 is detected by a pressure sensor 2, thereby sampling combustion pressure data at high speed so as to carry out several samplings per 0.1° in the turning angle of a crankshaft 3 by an analog-to-digital converter 55. A top dead center in the crankshaft 3 is detected by a turning angle sensor whereby a top dead center signal is outputted. With the control of a central processing unit 51, the sampling data of the A-D converter 55 and the top dead center signal both are stored in a random-access memory 53. Time series data are formed as the combustion pressure data of one piece per angle 1°, and these data are recorded in a memory card C via a memory card interface 56. Then the time series data are read out of this memory card C by a personal computer 6. Subsequently, both combustion pressure and differential diagrams are represented on a display by the personal computer 6.