The present invention provides a foam comprising (a) a polylactic acid resin, (b) at least one thermoplastic resin selected from a polyolefin resin and a polystyrene resin, and (c) a block copolymer of a vinyl aromatic compound and a conjugated diene, wherein a ratio of (a) to (b) by weight (a/b) is 80/20 to 20/80, and the foam comprises 0.5 to 20 parts by weight of (c) with respect to 100 parts by weight of (a) and (b) in total.