We have constructed a prototype dipole magnet. The field measurement has been performed. This paper reports the design concept and the results of the preliminary test about this magnet. The 3-GeV synchrotron proposed in the JAERI/KEK Joint Project is a rapid-cycling synchrotron (RCS), which accelerates a high-intensity proton beam from 400-MeV to 3-GeV at a repetition rate of 25 Hz. The 3-GeV synchrotron is used to produce pulsed spallation neutrons and muons [1]. It also works as an injector for a 50-GeV synchrotron. Since the magnets for the 3-GeV synchrotron are required to have a large aperture in order to realize the large beam power of 1 MW, there is a large leakage field at an end part than a usual synchrotron magnet. In addition, 25-Hz ac field induces an eddy current in magnet components: e.g. a coil, magnet end plates and etc. We intend to use a stranded conductor as a coil conductor so that the eddy current induced in the coil can be reduced. On the other hand, the eddy current induced in the end plates is expected to be large. Therefore, it is important to investigate effects of the large leakage field and the eddy field to the beam motion around the magnet end part. In addition, the eddy loss and the eddy field at the edge of the magnet caused by the eddy current was measured and compared to a simple two-dimensional model. This model supposes that time dependent external field is expressed by: ˆ it ext ext HH e ω