A linear guiding mechanism for a platform type optical scanner. A V-shaped track is installed inside a casing and positioned parallel to the travel path of a carrier chassis containing a system of optical devices. The upper section of the V-shaped track has a pair of support surfaces forming an included angle. The V-shaped track supports a positioning wheel or a positioning bump attached to the carrier chassis. The carrier chassis moves along the longitudinal direction of the V-shaped track when driven by a driving system. The V-shaped track may be constructed from a pair of monorails so that the driving belt may move inside the space between the monorails. An additional positioning structure may attach to the interior sidewall of the casing to serve as a retainer for the chassis in an intial position.