To provide a lock bolt for a blind screw hole to effect firm locking through simple operation without lowering the strength of a bolt and be used even for reuse without lowering workability.

A lock screw main body 6 having a tip with which a contact body 8 for lock is formed integrally is joined in a screwed-in state and coupled in a female screw hole 3 formed through the shaft central line (c) of a bolt body 4 such that the rotation center of a contact body 8 for lock is situated in a position on an eccentric axis eccentric at 9 from the shaft central line (c). After an object is fastened against a base body having a blind screw hole by the bolt body 4, a contact body 8 for lock is strongly pressed against a cup-shaped bottom situated deep in the blind screw hole of the object by operating the operation means 5 part of a male screw 7 for lock. Elastic bend deformation is applied on the male screw 7 for lock in a radial direction reverse to an eccentric direction and a double nut function is formed to lock a fastening state.