Fixing unit and image forming apparatus

A fixing unit includes a film with a tubular shape, a nip forming unit including a heater and configured to be in sliding contact with an inner surface of the film, the heater including a substrate made of metal, an insulating layer formed on the substrate, and a heating element arranged on the insulating layer and configured to generate heat when a first AC voltage is applied from an AC power supply connected thereto, a pressing member opposed to the nip forming unit with the film interposed therebetween and configured to form a nip portion with the film, and a voltage application circuit configured to apply a second AC voltage to the substrate with a waveform that takes a voltage value of an opposite polarity to the first AC voltage when the first AC voltage takes a peak value.