PURPOSE: To provide a lock mechanism by which folding of a handle of a bicycle and extending and contracting of a handle post can be locked, and the lock can be released by a single lever operation.

CONSTITUTION: A handle post 1 is provided with a structure in which an inner post 1b is fitted in an outer post 1a so as to be housed and can be locked by a piece 6. A handle bar 5 is pivotally attached on the upper end of the inner post 1b through a rotational hinge so as to be rotated and inclined sideways, the inner post 1b can be locked and unlocked by the piece 6 through an interlocking mechanism of links 13, 14 and a rod 16 by a release lever 7 provided on the upper end of the inner post 1b so as to be vertically turned, and a toggle joint 8 for locking the handle bar 5 to the inner post 1b in the normal attitude is provided. A protrusion 9 for releasing lock of the toggle joint 8 is projectingly provided on the outer post 1a.