To provide a main signal selection device for an ATM that can periodically monitor a switching function without having effect on a normal operation.

A traffic monitor section 3 monitors traffic of an input signal and a monitor control section 5 gives constant-periodically an insertion control signal INS to pattern insertion sections 6a, 6b. Respective specific monitor patterns are inserted to the pattern insertion sections 6a, 6b, which output signals S6a. S6b. The monitor control section 5 outputs a selection control signal CON to a changeover section 7 synchronously with the insertion control signal INS. Thus, the changeover section 7 outputs a combination of the first half and the 2nd half of the respective monitor patterns inserted to the pattern insertion sections 6a, 6b as a signal S7. A pattern inspection section 8 inspects a pattern of the signal S7 to monitor the changeover function of the changeover section 7.