PURPOSE: To carry out detection of an absolute position of a movable member in a power slide without causing complication of constitution.

CONSTITUTION: At output ends of reed switches 12-1, 12-10 of a branch circuit to a series circuit, their output voltage is monitored. Thereafter, a series of the reed switches is fixed on an upper rail 20 and a magnetic body 14 having the same magnetic pole width W2 as an arrangement pitch W1 of the reed switch is fixed on a lower rail respectively, the reed switch which works ON as the magnetic body approaches is discriminated by the output voltage from the branch circuit, and a seat position is detected from the position of the reed switch.