A planar rotation mechanism of supporting device including a base seat; a supporting rack; a retaining member having a first, a second and a third guiding slots with different curvatures and lengths; and a rotating member respectively provided with a column hole with respect to initial positions of the first, the second and the third guiding slots for allowing a first, a second and a third guiding columns respectively pass through corresponding column holes and corresponding guiding slots. The guiding columns can respectively slide within the corresponding guiding slots. When the rotating member is operated with a planar rotation with an object to be supported provided in front of the rotating member, the plural guiding columns slide within the corresponding guiding slots. The object to be supported is selectively switchable between a horizontal status and a vertical status.