The invention relates to an apparatus for heating a sheet- or web-like material during its transport through a processing machine, comprising at least one infrared heating panel facing the path of transport of the material and connected to an ac-source through semiconductor switching means, a controlling circuit for delivering ignition impulses to the semiconductor switching means, wherein the ignition time within each half period of the supply voltage is determined by a control signal provided by a control means to the controlling circuit, while the heating panel is switched off if the transport speed of the material becomes smaller than a minimum speed.