PURPOSE: To prevent adverse effect on charged particle beams by forming a bypass circuit as good conductor outside a specified part of a vacuum duct to shunt a vacuum duct current.

CONSTITUTION: As a charged particle beam passes inside a vacuum duct 11, a current Iw1 flows through the duct 11 with a size thereof the same as a current 1b carried by the charged particle beam but different in direction. In a current transformer 14, the existence of a bypass circuit 13 makes a part of current Iw2 flow through a specified part 11a of the duct 11 and the remaining current Iw3 flows through the circuit 13. And a phenomenon desired to be measured with the transformer 14 is often at a frequency band lower than a surrounding frequency of a circular accelerator. In the area with such a low frequency, the currents Iw2 and Iw3 are regarded as in a simple proportional relationship and a simple proportional relationship approximately is noted between the current Ib-Iw2 detected with the transformer 14 and the current to and thus, a charged particle beam current can be determined based on a detection signal of the transformer 14 by calibrating this relationship experimentally beforehand.