To provide an assembled mattress solving a conveyance problem and minimizing a sense of discomfort about a level difference caused between adjacent mat parts.

The assembled mattress includes an upper mat A, an under mat B and a mattress cover. The upper mat includes a mat body 10 divided in three, and edge mats 20 formed of outer frame bodies of elongated rectangular parallelepiped shape divided in three, and arranged to fit to both right and left sides of the mat body. The under mat B is formed by connecting mat pieces divided in four to be foldable into four. In the stacked state of the upper mat and under mat, the assembled mattress is constituted such that the positions of adjacent boundary lines a, b of the respective mats constituting the mat body, adjacent boundary lines c, d, e, f of the individual outer frame bodies constituting the edge mats, and adjacent boundary lines g, h, i of the respective mat pieces of the under mat do not overlap with one another.