Vehicle seat of type having upwardly foldable seat cushion

A vehicle seat having an upwardly-foldable seat cushion that includes an engagement piece projecting from the rear end of the seat cushion and an under cover below the seat having an upper panel portion having an upwardly sloped surface. A forwardly-and-rearwardly extending guide groove formed in that upper panel portion which allows the engagement piece to be moved therein, without its contact with the upwardly sloped surface of the upper panel portion, when the rear end of the seat cushion is slidingly brought to a lower end of a set back. Ribs are formed in a bottom surface of the guide groove. The guide groove allows forward-and-rearward movement of the engagement piece therein, so that the engagement piece can be pressed against and frictionally engaged with the lower end of the seat back, without being caught by the upwardly sloped surface of the upper panel portion.