Provided is an ethyleneimine polymer which exhibits excellent close adhesion properties and adhesive properties and has a low water content, and a method for producing the same.An ethyleneimine polymer, in which the number average molecular weight (Mn) of the ethyleneimine polymer measured by gel permeation chromatography in terms of pullulan is 13,000 or more and the value obtained by dividing the weight average molecular weight by the number average molecular weight (weight average molecular weight: Mw/number average molecular weight: Mn=degree of dispersion) of the ethyleneimine polymer is from 1.4 to 3.0.