Disclosed is an apparatus for adjusting the height of an armrest, capable of adjusting the heights of the arm rests arranged at the left and right sides of a driver's seat, respectively, in the vertical direction according to the physical condition of the driver. According to the present invention, the apparatus for adjusting the heights of the armrests arranged at the left and right sides of the driver's seat of a construction machine, respectively, comprises: an inner rail which is mounted on the driver's seat and in which joint protrusions are formed on a guide rail unit such that the joint protrusions maintain equal spacing in the vertical direction; an outer rail which is coupled to the inner rail such that the outer rail is slidable in the vertical direction, and which is mounted on the side surfaces of the armrests; a locker which is rotatably mounted on the outer surface of the outer rail, and which has, at the lower end thereof, locking grooves into which the joint protrusions are to be fitted; an adjustment lever which is fixed at the locker, and which presses a handle unit so as to enable the joint protrusions to be withdrawn from the locking grooves when the locker rotates about a fixing pin; and a connection member for connecting the lower end of the locker to the lower end of the outer rail.