An envelope filling apparatus inserts a sheet of paper into an envelope having a flap, and includes an envelope feed device, an air nozzle for blowing air into the envelope fed by the envelope feed device from an open end of the envelope to open the open end, and a sensing device for detecting that the open end of the envelope is opened. The apparatus further includes an insertion device for inserting the sheet of paper into the envelope from the open end, if the sensing device detects that the open end of the envelope is opened after the air nozzle is actuated, and a removal device for removing the envelope from a transport route, if the sensing device does not detect that the open end of the envelope is opened after the air nozzle is actuated and the insertion device is not actuated.