To attain a slide mechanism of a portable terminal device which, having a minimal number of parts and keeping the same function as that of a conventional one, enables a substantial production cost reduction, a mechanism relatively slides a first casing and a second casing in a state that the casings are laid over each other in a vertical direction, and is constituted with: a slide case mounted on either one of the first casing and the second casing; a slider slidably accommodated in the slide case and mounted on the other one of the first casing and the second casing; and a resilient means resiliently disposed between the slider and the slide case. The slide case is formed as an integral structure with a guide groove which extends to both ends in a longitudinal direction with the one end thereof being opened. Additionally the slider has a shape which enables the slider to be inserted into the guide groove in an engaged state from the open end side. Also, the slide case is further provided with a stopper portion preventing the slider from coming off after the slider is inserted into the guide groove.