A printer 1 including an image forming portion using toner containing a wax, a transfer portion for transferring an image onto a sheet, a belt unit and a pressing roller which heat the sheet in a nip, a duct including an air suction port for sucking air from a sheet inlet, a filter provided on the duct and for collecting dust, and a fan provided on the duct and for sucking the air includes a detector for detecting a temperature in the neighborhood of the belt unit and a control circuit portion for effecting control so that an air flow rate of the fan is weakened in the case where the temperature in the neighborhood of the belt unit is high during an image forming process and so that the air flow rate of the fan is weakened in the case where the temperature is high.