a frame body unit including a lower frame structure and an upper frame structure, said upper frame structure including a sliding rod that is slidably disposed on said lower frame structure, said upper frame structure being slidable in a direction toward a bottom of said lower frame structure, via sliding connection of said sliding rod and said lower frame structures, and fold on said lower frame structure, so as to convert said frame body unit from an unfolded state to a folded state; and

two wheel units disposed respectively on two sides of said lower frame structure and operable to pivot toward said lower frame structure;

wherein said lower frame structure includes a bottom seat, two lateral rods, and a stationary rod connected between said lateral rods, said upper frame structure further including a main frame body and two engaging rings, said main frame body having a handle portion and two side frame portions, said engaging rings being disposed respectively on said side frame portions, said engaging rings being engaged to said stationary rod when said frame body unit is in said unfolded state, and being driven respectively and rotatably by said side frame portions to disengage from said stationary rod to thereby allow conversion of said frame body unit from said unfolded state to said folded state.