To accurately detect an original boundary line between black and white colors even if there is an unstable reflected color other than black and white within the range of detection when the black and white boundary line of a target used for positioning is accurately detected by using a laser sensor regarding an accessor positioning for a magnetic tape drive in a magnetic tape library system.

A target 1 including a black portion 11 (dimension A) and a white portion 12 (dimension B) is attached to a magnetic tape drive. A laser sensor is attached to an accessor mechanism, a laser light 2 emitted from the laser sensor is swept from an R side to an L side within a laser reading range (dimension X) and a boundary line 13 is detected. If the dimension of a gray area 15 from the starting point of reading a laser light to the black portion is C, a dimension relation is set to satisfy (A+B)C.