To reduce the cost of manufacturing and setting of a protective cover for a jumping type auxiliary seat to be set up in a microbus, a one-box car or the like and simultaneously to make improvements in fitting stability, aside from being good in appearance.

A mounting piece 31 and a hinge part 32 are set up in an upper edge of a single protective cover 30. A tension spring 35 drawing and energizing the protective cover 30 is installed at the linkage side, and a guide rib 33 with a cam function coming into contact with a tip edge 241 of a frame 24 is installed in an inner surface. In response to the jumping operation of a seat cushion 21, the protective cover 30 is made to be followed up into operation in a state that it is closely adhered to the linkage side at all times. In the jumping state, an engaged claw 34 on an inner surface of the protective cover 30 is operated to make a spring force of the tension spring 35 act in the engaging direction of an engaged hole 242 of the frame 24.