Automatically extensible and retractable wheel unit for mounting on a corner of a suitcase

The automatically extensible and retractable wheel unit for mounting on a corner of a suitcase includes a housing with an aperture at the underside thereof, a bifurcated bracket pivotally supported on the housing, a wheel carried rotatably on the bracket for pivoting along with the bracket between a retracted position and an extended position, and a push-pull member pivotally supported on the housing and connected to a bracket by a C-shaped spring member. When the wheel is in the retracted position, a portion of the push-pull member extends out of the aperture so as to be contacted with the ground to push the wheel automatically out of the aperture as long as the suitcase is brought into a transported state. When the wheel is in the extended position, another portion of the push-pull member extends out of the aperture so as to be contacted with the ground to pull the wheel automatically back into the aperture as long as the suitcase being in a stationary state.