Abstract: A papermaking additive comprising cellulose nanofibers whose aqueous solution at a concentration of 2% (w/v) has a B-type viscosity (60 rpm, 20° C.) of 500 to 7000 mPa·s, preferably 500 to 2000 mPa·s is used to prepare a paper having improved air resistance and smoothness. This additive is prepared by oxidizing a cellulosic material with an oxidizing agent in the presence of (1) N-oxyl compound(s) and (2) bromide, iodide or a mixture thereof to prepare oxidized cellulose, and finely grinding the oxidized cellulose in a wet condition to convert the oxidized cellulose into nanofibers.