In a mechanism which relatively slides a first casing and a second casing of a portable terminal device in a state that the casings are laid over each other in a vertical direction, in order to attain a slide mechanism which is simply structured and structured to enable open/close operation of the first casing and the second casing to be automatically conducted from predetermined slide positions of the both casings, the mechanism relatively slides the first casing and the second casing in the state that the casings are laid over each other in the vertical direction, and is constituted with: a base member mounted on either one of the first casing and the second casing; a slider slidably mounted on the base member and mounted on the other one of the first casing and the second casing; and a resilient means, which is disposed between the slider and the base member, for slidably biasing the slider in a closing direction and/or an opening direction from predetermined slide positions, by being displaced according to a slide position of the slider. Meanwhile, the resilient means is a torsion spring and the torsion spring is mounted so that a direction of action of resilience thereof is displaced according to slide operation of the slider, by one end side of the torsion spring being pivotally supported on the base member and the other end side being pivotally supported on the slider.