A rapid-cycling magnetic field is necessary to operate a synchrotron with a high repetition rate for the 3 GeV proton synchrotron of JAERI-KEK joint project. Since an eddy-current effect in a metal duct would produce an unacceptable perturbation of the magnetic field and unnecessarily large ohmic losses, an alumina ceramic is a candidate material for vacuum ducts in the dipole and quadrupole magnets. The required beam apertures, being a circular cross section, are 187 and 247 mm in diameter for the vacuum ducts to be installed in the dipole and quadrupole magnets, respectively. The duct lengths for each magnet are 3.5 and 1 m. Since such a long duct is difficult to make with accuracy in the radial dimension, when sintered at once, it can only be manufactured by jointing duct segments of about 0.5 m long. The mechanical strength and deterioration under a high level of radiation were examined for various kinds of alumina ceramic and for the joint. Almost all of the ceramics to be tested have a sufficiently high flexural strength of 300 MPa, or more, before irradiating with a 2.5 GeV-electron beam, and they did not show any significant deterioration in the flexural strength as far as an absorption dose is up to 1000 MGy.