A weight control apparatus of a weight training machine, the weight control apparatus comprising: a plurality of stacks which are fitted to a pair of guides standing upright between supports on a base such that the stacks slidably move up and down, and which have rectangular locking holes; a body arranged on a front surface of the base, in which the body has a plurality of insertion holes corresponding to the locking holes of the stacks and is provided at an upper portion thereof with a holder; and a cover arranged on the front surface of the body and having a plurality of locking pin members inserted into the insertion holes, wherein each of the locking pin members include: a rectangular stopper having a recess inwardly recessed, protrusions protruding fromboth sides of the recess, and locking protrusions (515-1, 515-2) protruding from both sides of the stopper; a shaft pin connected to a front end of the stopper; a button arranged on a front end of the shaft pin; and a conical coil spring arranged between the button and the shaft pin, and wherein the body is provided therein with a locking member, the locking member locking or unlocking the locking protrusions (515-1, 515-2) and the locking member includes a plate member having a predetermined length and provided at an upper end thereof with a spring, and a locking protrusion protruding from one side of the plate member, and the locking protrusion includes:a protrusion having a trapezoidal shape and formed with a first inclined surface inclined downward and an incised surface horizontally incised in adjacent to the first inclined surface;a recess which is inwardly recessed and disposed below the first inclined surface;a second inclined surface disposed below the recess and inclined in a direction identical to an inclination direction of the first inclined surface; and a groove adjacent to the second inclined surface, characterized in that a safety guide is installed in the body in opposition to the locking member, the safety guide including a plate member, a plurality of locking protrusions and a plurality of grooves, the plate member having a predetermined length and provided at an upper end thereof with a safety spring, the locking protrusions and grooves being alternately formed in a regular interval at one side of the plate member. a plurality of stacks which are fitted to a pair of guides standing upright between supports on a base such that the stacks slidably move up and down, and which have rectangular locking holes; a body arranged on a front surface of the base, in which the body has a plurality of insertion holes corresponding to the locking holes of the stacks and is provided at an upper portion thereof with a holder; and a cover arranged on the front surface of the body and having a plurality of locking pin members inserted into the insertion holes, wherein each of the locking pin members include: a rectangular stopper having a recess inwardly recessed, protrusions protruding fromboth sides of the recess, and locking protrusions (515-1, 515-2) protruding from both sides of the stopper; a shaft pin connected to a front end of the stopper; a button arranged on a front end of the shaft pin; and a conical coil spring arranged between the button and the shaft pin, and a rectangular stopper having a recess inwardly recessed, protrusions protruding fromboth sides of the recess, and locking protrusions (515-1, 515-2) protruding from both sides of the stopper; a shaft pin connected to a front end of the stopper; a button arranged on a front end of the shaft pin; and a conical coil spring arranged between the button and the shaft pin, and wherein the body is provided therein with a locking member, the locking member locking or unlocking the locking protrusions (515-1, 515-2) and the locking member includes a plate member having a predetermined length and provided at an upper end thereof with a spring, and a locking protrusion protruding from one side of the plate member, and the locking protrusion includes:a protrusion having a trapezoidal shape and formed with a first inclined surface inclined downward and an incised surface horizontally incised in adjacent to the first inclined surface;a recess which is inwardly recessed and disposed below the first inclined surface;a second inclined surface disposed below the recess and inclined in a direction identical to an inclination direction of the first inclined surface; and a groove adjacent to the second inclined surface, characterized in that a safety guide is installed in the body in opposition to the locking member, the safety guide including a plate member, a plurality of locking protrusions and a plurality of grooves, the plate member having a predetermined length and provided at an upper end thereof with a safety spring, the locking protrusions and grooves being alternately formed in a regular interval at one side of the plate member.