var targetPos:Number = _root.flag._x;
// 残り距離に対する進む距離の割合
// 数値が大きくなるほど速く動く (0 < deceleration <= 1)
var deceleration:Number = 0.2;

this.onRelease = function():Void {
    this.onEnterFrame = function():Void {
        this._x += (targetPos - this._x)*deceleration;
        if (Math.abs(targetPos - this._x) < 0.5) {
            this._x = targetPos;
            delete this.onEnterFrame;
        }
    };
};