In this paper, we attempt to estimate lead time (duration) of each period of an operation process by a factory worker using a wrist-worn accelerometer. In a factory line production system, a worker repetitively performs predefined operation processes, and the lead time greatly affects productivity of the line production system. Our proposed method automatically finds a frequent sensor data segment as a “motif” that occurs once in each operation period using only prior knowledge about predefined standard lead time of the operation process, and uses the occurrence intervals of the motif to estimate the lead time.