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PROBLEM TO BE SOLVED : To provide a continuously cast slab of steel in which crystal grains are fine, and a continuous casting method therefor. SOLUTION : There is provided a continuously cast slab of steel comprising 0.0001 to 0.005% Ag and further comprising one or two kinds selected from 0.001 to 0.01% rare earth elements (REM) and 0.001 to 0.01% Zr, and in which Ag%/(REM%+Zr%) satisfies 0.1 to 0.4, and the maximum circle equivalent diameter of inclusions including the REM or/and Zr and Ag is 5 μm or lower. In this way, the crystal grain size of the continuously cast slab can be refined to below 5 mm. The continuously cast slab can be produced by adding the lump of a rare earth element(s) and Zr to molten steel in an RH vacuum tank or in a ladle, and thereafter adding an Ag-involving iron-covered wire or Ag grains into a tundish or a mold. SELECTED DRAWING : None COPYRIGHT : (C)2018, JPO&INPIT