has been thermally expanded is then sent to a drying step for drying although an ordinary drying method of the prior art may be used for drying it is essential to avoid crushing the sheet with a strong press the temperature of the moist hot air or water vapor used for the invention may be above the temperature at which the microcapsule shell walls of the heat expanding particles soften and begin to expand and it will be determined by the heat expanding particles used the relative humidity is preferably 100 in order to prevent drying of the wet mixed sheet during the thermal expansion step but it does not necessarily need to be 100 the method of supplying the moist hot air or water vapor is most preferably a method in which high temperature steam from a boiler is ejected and directly sprayed onto the sheet but moist exhaust from the drier may also be used the density of the bulky paper of the invention is at least 0 01 g cm 3 and less than 0 1 g cm 3 and preferably at least 0 01 g cm 3 and no greater than 0 05 g cm 3 the density of the bulky paper of less than 0 01 g cm 3 is not practical because the strength.