To provide a fire resistant device and a fire resistant method which can be easily constructed while improving fire resistant property by increasing packing density of a fire resistant material filled in a gap formed between a floor and an outer wall and which can flexibly block an interlayer even when sizes of gaps formed between a floor and an outer wall change depending on places.SOLUTION: A first core material 111 inserted along a longer direction of a gap comprises an incombustible material. A second core material 112 which is provided at least one side of a floor side and an outer wall side on a lateral face in a longer direction of the first core material 111 and which blocks the gap comprises an elastic incombustible material with an elasticity higher than that of the first core material 111.SELECTED DRAWING: Figure 1