present purpose as such heat expanding particles there are known matsumoto microsphere f 36 f 30d f 30gs f 20d f 50d and f 80d product of matsumoto yushi seiyaku co ltd and expancel wu and du product of sweden marketed by japan fillite co ltd although there is no limitation to these the heat expanding particle content is 1 40 parts by mass and preferably 3 20 parts by mass with respect to 100 parts by mass of the pulp fiber because at less than 1 part by mass the expansion will not be sufficient while economical disadvantages are presented at greater than 40 parts by mass the pulp slurry may further contain various anionic nonionic cationic or amphoteric yield improvers paper strength additives sizing agents and the like selected as appropriate specifically as paper strength additives and yield improvers there may be used combinations of organic compounds such as polyacrylamide based cationic nonionic anionic and amphoteric resins polyethyleneimine and its derivatives polyethylene oxide polyamines polyamides polyamidepolyamine and its derivatives cationic and amphoteric starch oxidized starch carboxymethylated starch vegetable gum polyvinyl alcohol urea formalin resin melamine formalin resin and hydrophilic polymer particles and inorganic compounds including aluminum compounds such as aluminum sulfate alumina sol basic aluminum sulfate basic aluminum chloride and basic polyaluminum hydroxide and iron ii sulfate iron ii chloride colloidal silica bentonite or the like in the paper making process of the invention the starting slurry obtained by mixing within water in the prescribed proportions is sheeted with a wire part and.