An image generating apparatus is provided which can maximize the throughput without causing thermal damage of the fusing components. It determines a sub-thermistor temperature threshold value for switching small size paper feed intervals in response to a sub-thermistor initial temperature. It supplies a heating body with current for heating, and starts feeding recording mediums at the feed interval determined by a feed interval initial value. It makes a successively conveyed paper count. Every time the successively conveyed paper count reaches a paper count threshold value, it performs the switching control of the sub-thermistor temperature threshold value. If the sub-thermistor temperature exceeds the threshold value while the recording medium is passing through the fusing apparatus, it carries out the switching control of the feed intervals. In another mode, it prevents the extension of the paper feed interval of the paper conveyance during a specified paper count .alpha..