To provide for gradual decay of current flow through the primary of an ignition coil, and hence suppression of spurious sparks at spark plugs, a main switching transistor has its conductivity controlled by the conductivity of the emitter-collector path of a control transistor, the base-emitter path of which forms part of a discharge circuit of a control capacitor, which discharge circuit has a sufficiently high resistance to provide for gradual discharge of the capacitor and hence gradual transition of the conductivity of the control transistor.