A method for compressing or decompressing a spring loading a tie rod in a jaw crusher. The tie rod is supported by, at one location, a pendulum of the jaw crusher and, at another location, the jaw crusher's frame through the said spring. The spring is installed between the jaw crusher's frame and a second spring support supported by the tie rod; and the second spring support is moved in relation to the tie rod. A support part of the tightening device and at least two moving screws supported by the support part are arranged to influence the second spring support. The support part is supported by the tie rod through a locking member; and the at least two moving screws supported by the support part are moved to change the distance between the second spring support and the support part.