PURPOSE: To enable quick data analysis by a method wherein a maximum combustion pressure (Pmax) stored with a peak holding circuit is read and stored into a sample holding circuit using a top dead point cycle of an exhaustion stroke of a cylinder as sampling time to determine a value of the P max simply in a real time.

CONSTITUTION: With the rotation of a crank shaft 24 of an engine 10, a combustion pressure signal (a) of a cylinder 11 generated with a combustion pressure sensor 12 is inputted into a peak holding circuit 21 through a charge amplifier 18 and the circuit 21 reads and stores a maximum Pmax (c) of the combustion pressure signal with a timing t1 near an upper dead point of a compression stroke of the cylinder 11. A sample holing circuit 22 reads and stores (d) the Pmax of the circuits 21 at a rise timing t2 of a pulse with a top dead point cycle of an exhaustion process of the cylinder 11 as sampling time. Thus, the Pmax per combustion can be outputted from the circuit 22 as a cam shaft 14 makes each rotation thereby calculating maximum and minimum of the Pmax all through the combustion on the same operational conditions with a personal computer 23.