To provide a fixing device which can firmly fix a clamp supported by receiving projections mounted to the outer periphery of a single pipe of a scaffold main column, used when erecting a temporary scaffold in a construction site, at predetermined intervals to a support.

A clamp 3 which is mounted to a scaffold main column 1 is mounted with: a bearing part 11 which is installed on a long piece mounting part 10 of a fixed part 5; and an engaging key rod 12 which rises and falls, and rotates. A movable part 6 facing the fixed part 5 is mounted with a locking fitting 8 having a tubular shape or the like, on the inner surface of a mounting piece 7. The clamp 3 is easily fixed to the scaffold main column 1 by engaging the engaging key rod 12 with the locking fitting 8.