To provide a method of using a foamed flame-retardant covering material which positively prevents the entry of flames through a gap of an opening.

The foamed flame-retardant covering material is applied to a flame entering portion from which flames enter in case of fire, and its foaming and filling function is achieved at a temperature below 240 °C. The flame entering portion may be a gap between plates of plate-like wall forming members, a joint portion between the same, a venting portion in a door, a cavity between the door and a door frame, a cavity between a window and a window frame, a vent hole, a ventilating opening, an intake port, an exhaust port, and an inner cavity of a cable conduit. The application of the foamed flame-retardant covering material to the flame entering portion is implemented by coating the material or by sticking a sheet of the same.