A base structure of a turnout, which has high durability and is capable of reliably preventing lateral displacement, is disclosed. The base structure of the turnout comprises a set of steel sleepers 1A, which is installed on the track bed and can be filled with ballast. The set of steel sleepers 1A comprises a plurality of steel sleepers 2a, 2b, 2c having a substantially same configuration and different lengths, which are arranged in parallel to each other at predetermined intervals with the length varying stepwise from the shortest steel sleeper 2a to the longest steel sleeper 2c, with end faces on both sides of the longitudinal direction of all steel sleepers are capped with sleeper tie members 3, 4, and 5 or 6, and a portion, or all, of the sleeper tie members are embedded in the track bed.