PURPOSE: To enable easy insertion of a lock pin into an open hole bolt having a cross hole formed by aligning optical axes of light projecting and receiving units with the through hole of a socket and further aligning the optical axes to the cross hole of the bolt.

CONSTITUTION: A spindle 24 is provided with a socket 26 formed on its tip and holding a nut with groove 2 so that the nut can oppose to an open hole bolt 1 which is secured. A rotary cylinder 16 rotatably supports the spindle 24 and a ratchet wheel 31 is integrally rotated with the spindle 24. A torque motor 36 rotates the nut with groove 2 in the clamping direction in relation to the open hole bolt 1 and an actuator rotates the rotary cylinder 16. Light projecting and receiving units 47, 48 are provided on the tip of the rotary cylinder 16 and a ratchet piece 40 is provided on a portion of the rotary cylinder 16 so as to engage with the ratchet wheel 31. An encoder 45 is provided to detect rotating angle of the spindle 24.