A carriage driving unit comprises a carriage, an timing belt, an timing belt, which is jointed to the carriage, looped around between a driving pulley attached to a driving shaft of a motor and a driven pulley attached to a driven shaft, a frame for rotatably supporting said driving shaft in a cantilever manner and a regulating projection for regulating the timing belt from displacing in a direction closer to said frame with respect to said driving pulley.