To avoid the influence of attenuation of output of an integration circuit caused due to discharge of a capacitor, in a combustion state detecting device wherein a signal processing circuit integrates an output value of a sensor corresponding to change speed of pressure in a combustion chamber with the capacitor and the pressure in the combustion chamber is obtained based thereon.

In an ECU 3 receiving an output signal of the signal processing circuit 2, a multiplier value obtained by multiplying a previous sampling value by a coefficient is subtracted from a current sampling value of the output signal, and processing for defining this value as change quantity of the pressure in the combustion chamber is executed so as to obtain the change quantity in the combustion chamber without being influenced by the attenuation.