A vaccine based on live Arthrobacter cells is useful in preventing piscirickettsiosis in fish. The first and second devices may include multiple registers. The permanent magnet 3 whose product of a coercive force and a magnetizing direction thickness is small is irreversibly magnetized by a magnetic field created by a current of an armature coil 21, to change a total linkage flux amount. Further, on the surface of the case 1, which corresponds to one end side of the modules M, a second discharge opening 10a is formed for discharging open air introduced into the second space 8a from this second introduction opening 9a after this open air has passed the inside of the second space 8a.