Provided is a water absorbent sheet retaining mat which can retain a water absorbent sheet easily and surely. A water absorbent sheet retaining mat for retaining a water absorbent sheet having a first polygonal contour, includes: a mat member having a second polygonal contour defined by mutually opposing a first main plane and a second main plane; and at least one retaining member having a slit member including at least one slit formed in a top surface at a first predetermined height from the second main plane for accepting a part of perimeter of the water absorbent sheet being inserted therein.