A measuring circuit for the ionic current measurement in the combustion chamber of an Otto engine is disclosed. The circuit has an ignition transformer the secondary coil of which supplies an ignition circuit. On the side carrying the positive voltage of the ignition pulse, the coil is connected to ground and, on the side carrying the negative voltage of the ignition pulse, it is connected to the ignition spark gaps via a distributor. The circuit has, inserted into the ignition circuit, a parallel circuit of a high-voltage Zener diode connected in the non-conducting direction related to the ignition current, and a capacitor as the measuring voltage source. The present invention supplies the measuring voltage to the ignition spark gap without circuit elements influencing its sensitivity. A parallel circuit is connected between the side carrying the positive voltage of the secondary coil and ground. The secondary coil is connected via diodes immediately to the ignition spark gaps such that the anode of each diode is connected to the side of the secondary coil carrying the negative voltage of the ignition pulse. A coupling capacitor for decoupling of an ionic current signal is coupled to the primary winding of the ignition transformer.