To provide a closed-cell cushioning sheet good in moldability, excellent in strength, etc., and having a high biodegradable speed.

In a closed-cell cushioning sheet formed by laminating an embossed film 2 having a large number of projected parts 3 formed to the entire surface thereof and a planar base film 1 and/or the embossed film 2, the embossed film 2 and the base film 1 are composed of polycaprolactam alone or a compsn. of polycaprolactam and an aliphatic polyester resin and the polycaprolactam is subjected to radiation treatment alone or along with at least other one constitutional component to set the gel ratio of polycaprolactam to 0.01-10%.