PURPOSE: To correct a reference point position error accurately with simple constitution, by connecting a connecting rod, which is relatively movable to and from a reference bar horizontally movable to and from a reference carriage and forms a reference point, to a correction wire fixed to the carriage.

CONSTITUTION: Together with slide bar 3 horizontally movable to and from reference carriage 1 at a curve part, etc., two shores 5, reference bar 9, etc., move. Both the ends of connecting rod 12 supported relatively movably by bar 9 through two guides 16 are coupled with correction wire 13 fixed to carriage 1 and never shift relatively from carriage 1 which shifts in position from rails. Consequently, the relative position relation between reference point 10 formed by rod 12 and rails will not vary even at the curve part, etc. Therefore, a reference point position error is substantially corrected with the simple constitution to provide easy and accurate track corrections, etc.