PURPOSE: To detect the peak position of a radiated light beam accurately.

CONSTITUTION: The monitor for detecting the peak position of a radiated light comprises a probe support 14 disposed in a vacuum chamber 10 in order to support a wire 15, and an air cylinder 12 for moving the probe support 14 such that the wire 15 traverses a radiated light beam SR passing through the interior of the vacuum chamber 10. The monitor further comprises a timer 20 interlocked with the air cylinder 12, and a peak position operating unit 22 for determining the peak position of the radiated light beam SR with respect to the vacuum chamber 10 based on a time elapse signal 21 outputted from the timer 20. Since the wire 15 traverses the radiated light beam SR at high speed, the wire 15 is not fused and the peak position of the radiated light beam SR can be detected correctly.