A seat erecting/folding structure having a simple structure and a low weight and being capable of advantageously preventing, when a movable spectators' stand is to be used in a state of being developed to an intermediate stage, spectators and the like from entering an extra developed deck. In the movable spectators' stand having a plurality of decks (1, 2) capable of being nested and developed and seats (3) arranged erectably on the decks (1, 2), legs (5) of each seat (3) are hinged to a base frame (4) of the deck (1), and a bent portion of a crank member (10) provided with a swingable arm (12) projecting toward above the base frame (4) and a hook (13) projecting forward from the lower end of the swingable arm (12) is pivoted on the base frame (4), the projecting length of the swingable arm (12) is such that the distal end comes into contact with the front face (2a) of the upper deck (2), and each leg (5) is provided with a lock member (9) engageable with the hook (13) in its upright position.