PURPOSE: To make a safety frame and covers attachable or detachable to or from a machine base free of rotation around the same axle in an easy manner, by installing the safety frame for protection of riders and symmetrical rear- wheel covers by means of a metal being fitted in the outer circumference of an axle housing.

CONSTITUTION: Symmetrical rear-wheel covers 2 are solidly attached to a safety frame 1 for protection of riders. And, a rear lower end part of the safety frame 1 is rotatably attached to the outer circumference of an axle housing 3 via a metal 4. At this time, the axle housing 3 projectingly installs symmetrical rear- wheel driving shafts 7 through bearings at the rear end of a machine bed 5, while symmetrical driving rear wheels 8 is attached to the outer end of a shaft 7 through bearings. Likewise, a setting seat 19 of a rear strut 15 in the safety frame 1 is installed in an upper part of the metal 4, then a tooth profile part 20 is installed in an inner part of the setting seat 19 and, in turn, a worm gear 21 is engaged with this tooth profile part 20. With this constitution, the setting of safety frame bodies 1 and 2 to the machine base 5 and vice versa is thus easily performable.