When a magnet wound with coated conductors generates a time-dependent magnetic field, ac losses are generated in coils of the magnet. We have developed a three-dimensional model for electromagnetic field analyses of multilayered coils composing a cosine-theta dipole magnet with an iron yoke. The influence of the iron yoke is considered as the image currents in the iron yoke. Using this model, we conducted electromagnetic field analyses of a dipole magnet, which was designed from the specifications of a rotating gantry for carbon cancer therapy. The time-dependent spatial distributions of ac losses were obtained from the results of electromagnetic field analyses when the magnet was excited at 20 and 30 K. The feasibility of conduction-cooled magnets for the rotating gantry is discussed from the viewpoint of the heat load by ac losses.