The present invention is an antimicrobial agent containing a polyalkyleneimine derivative formed by adding a substituent group having a structure of a following Formula (1) to a nitrogen atom of polyalkyleneimine. In the formula, R 1 represents an alkyl group with 6 to 20 carbon atoms, an alkenyl group with 6 to 20 carbon atoms, an aryl group with 6 to 20 carbon atoms, or -(CH 2 CH 2 O) n -R 2 . R 2 represents an alkyl group with 6 to 20 carbon atoms, an alkenyl group with 6 to 20 carbon atoms, or an aryl group with 6 to 20 carbon atoms. n represents an integer of 1 to 50.