PURPOSE: To monitor the state change at a short scanning period without oversight and reduce the storage capacity of the memory, by detecting state display data of many channels, which are monitored concentratedly, by the block unit of every plural channels.

CONSTITUTION: Digital signals indicating turn-on/off states of systems to be controlled of many channels are distributed to (n) routes consisting of (m) blocks for every (n) channels, and the on/off signal of each route is switched successively by circular changeover switch 2 and is applied to input port switching part 3. M-block signals of respective routes from switching part 3 are collected for every (n) blocks corresponding to two new and old channel periods and are supplied to on/off detecting part 4 and off/on detecting part 5 in parallel. Signals of state display data of channels changing from turn-on to turn-off and changing from turn-off to turn-on are output from detecting parts 4 and 5 respectively and are registered in state change register part 7. Fundamental characters are combined with information characters of state change by character editing part, and they are recorded.