Abstract: A method for starting continuous slab casting capable of facilitating withdrawal of a dummy bar to start smooth continuous slab casting is provided at the start of casting of continuous slab casting, where lateral side plates 2 of the mold 10 are pinched by longitudinal side plates 1 of the mold and are adjustable in the slab width direction; where a dummy bar 4 is set at a desired level at the center in a width-variable, continuous casting mold; and clearances each between lateral side plates 2 and the dummy bar 4 are adjusted to 1-3 mm, while maintaining the lateral side plates 2 vertically, then sealing materials 5 are filled into the clearances; then molten steel is poured into the mold; and then the top ends of the lateral side plates 2 are made more apart outwardly, thereby giving a downwardly tapered profile to the lateral side plates 2, when no more poured molten steel is passed into the clearance between the formed solidified shell 6a and the plates 1 and 2, while withdrawing the dummy bar 4 downward