In the case where multiple individual work devices respectively executing a sequence of multiple tasks are controlled in an integrated manner by an integrated control device, log analysis among the multiple individual work devices is difficult. One of four individual work devices transmits time inquiry signal to main integrated control device via serial communication cable. Main integrated control device returns the time at time at which time inquiry signal was received to the one of four individual work devices to which time inquiry signal was transmitted. One of four individual work devices receives a reply from main integrated control device at time, adds the communication time to the time at time, and calculates the time at time. One of four individual work devices starts the calculation of the in-device time from calculated time as a starting point. One of four individual work devices logs based on in-device time.