Vehicle seat including: a seat back pivoted to a side of floor; a seat cushion pivoted to the seat back; and a front leg element having an upper end pivoted to the seat cushion and a lower end pivotally locked to the side of floor in a releasable manner, thus normally providing a four-pivotal-point linkage in the seat. Maintaining the four-pivotal-point linkage adversely restricts design and actions of the seat. Inequality conditions are therefore set in that four-pivotal-point linkage, such that folding down the seat back by an angle exceeding a predetermined angle results in unlocking of the lower end of the front leg element from the side of floor, thereby destroying the four-pivotal-point linkage. Hence, a freedom of design is increased for the seat and floor. Further, leverage mechanism(s) may be provided to facilitate the ease of unlocking the front leg element from the side of floor.