Abstract: Disclosed is a mattress comprising a flat-constructed spring assembly, a pair of elastic material sheets which are attached to upper and lower faces of said spring assembly and whose peripheral edge portions are so extended as to form a space at a side peripheral portion of said spring assembly and opposed to each other at their respective tip ends, and a side-elastic material member formed in said space by subjecting a foamable plastic to foaming-in-place, and a covering means for covering an elastic body consisting of said spring assembly, said elastic material sheets and said side-elastic material member, whereby to eliminate a feeling of physical disorder during the use of the mattress and to prevent a mattress body from being subject to lateral swing or rocking motion.