A slide unit mechanism for a portable electronic device, comprising: a quadrangular first fixed plate including a protrusion formed in a middle of a back surface thereof along a sliding direction and recesses formed on both sides of the protrusion; a sliding plate configured to slide on the first fixed plate between an open position and a closed position, the sliding plate including slide parts that slidably engage edges of the first fixed plate and a connecting part that connects the slide parts on a front surface of the first fixed plate; a second fixed plate having a size greater than the first fixed plate, wherein the first fixed plate with the sliding plate attached thereto is attached to the second fixed plate; and sliding springs disposed in the recesses and between the first fixed plate and the second fixed plate, first ends of the sliding springs being held at positions substantially in a middle of the first fixed plate in the sliding direction and second ends of the sliding springs being held by the slide parts; characterized in that it further comprises rattle preventing mechanisms configured to engage the sliding plate when the sliding plate is slid to at least one of the open position and the closed position to thereby prevent the sliding plate from rattling, wherein each of the rattle preventing mechanisms includes a recess formed in a back surface of the second fixed plate at a position corresponding to one of the open position and the closed position of the sliding plate, an opening that passes through the second fixed plate from the recess to a front surface of the second fixed plate and is positioned on a moving path of one of the slide parts, a locking torsion spring disposed in the recess, one end of the locking torsion spring engaging the second fixed plate and another end of the locking torsion spring being attached to an engaging part that protrudes from the opening on the moving path of the one of the slide parts, and an engaging recess formed in the one of the slide parts and configured to engage the engaging part; wherein the engaging part is configured to engage the engaging recess and to be biased by elastic force of the locking torsion spring toward the engaging recess when the sliding plate is slid to the open position or the closed position. a quadrangular first fixed plate including a protrusion formed in a middle of a back surface thereof along a sliding direction and recesses formed on both sides of the protrusion; a sliding plate configured to slide on the first fixed plate between an open position and a closed position, the sliding plate including slide parts that slidably engage edges of the first fixed plate and a connecting part that connects the slide parts on a front surface of the first fixed plate; a second fixed plate having a size greater than the first fixed plate, wherein the first fixed plate with the sliding plate attached thereto is attached to the second fixed plate; and sliding springs disposed in the recesses and between the first fixed plate and the second fixed plate, first ends of the sliding springs being held at positions substantially in a middle of the first fixed plate in the sliding direction and second ends of the sliding springs being held by the slide parts; characterized in that it further comprises rattle preventing mechanisms configured to engage the sliding plate when the sliding plate is slid to at least one of the open position and the closed position to thereby prevent the sliding plate from rattling, wherein each of the rattle preventing mechanisms includes a recess formed in a back surface of the second fixed plate at a position corresponding to one of the open position and the closed position of the sliding plate, an opening that passes through the second fixed plate from the recess to a front surface of the second fixed plate and is positioned on a moving path of one of the slide parts, a locking torsion spring disposed in the recess, one end of the locking torsion spring engaging the second fixed plate and another end of the locking torsion spring being attached to an engaging part that protrudes from the opening on the moving path of the one of the slide parts, and an engaging recess formed in the one of the slide parts and configured to engage the engaging part; wherein the engaging part is configured to engage the engaging recess and to be biased by elastic force of the locking torsion spring toward the engaging recess when the sliding plate is slid to the open position or the closed position. a recess formed in a back surface of the second fixed plate at a position corresponding to one of the open position and the closed position of the sliding plate, an opening that passes through the second fixed plate from the recess to a front surface of the second fixed plate and is positioned on a moving path of one of the slide parts, a locking torsion spring disposed in the recess, one end of the locking torsion spring engaging the second fixed plate and another end of the locking torsion spring being attached to an engaging part that protrudes from the opening on the moving path of the one of the slide parts, and an engaging recess formed in the one of the slide parts and configured to engage the engaging part; wherein the engaging part is configured to engage the engaging recess and to be biased by elastic force of the locking torsion spring toward the engaging recess when the sliding plate is slid to the open position or the closed position.