An apparatus for detecting a misfire in an internal combustion engine that is capable of preventing erroneous detection in a period in which the discharged voltage on a secondary ignition coil is charged. The apparatus having a capacitor, which is supplied with bias voltage from a primary side of an ignition coil to be electrically charged to apply the charged voltage to the spark plug at the time of discharge of the spark plug to cause an ionic current to flow and a misfire detection circuit for determining whether or not a misfire has taken place in accordance with detection of the ionic current flowing from the capacitor. The apparatus for detecting a misfire in an internal combustion engine includes a discharge-period detecting Zener diode disposed between another end of the primary coil and an inverting input terminal of an operational amplifier, having Zener voltage lower than the Zener voltage of a Zener diode, which sets voltage to be charged into the capacitor, and connected in a direction in which an electric current flowing while exceeding the Zener voltage is caused to flow toward the inverting input terminal of the operational amplifier.