The present invention relates to a closed force transmission device in which an insertion depth of a latch bolt inserted into a latch bolt insertion groove can be adjusted to improve safety and components are simplified to allow for ease of assembly and to improve durability. The present invention also relates to a safety door lock using the device. The closed force transmission device of the present invention includes: a cylindrical body housing of which both sides are opened to penetrate a door; a main body slidably arranged within the body housing such that the main body moves in a straight line direction by the force applied from an external source, the main body having at least one tilt surface and a main body movement space formed orthogonally to the straight movement direction in a portion corresponding to the tilt surface.