The nonionic surfactant of the present invention has a structure represented by formula (1): wherein each of R1 and R4 is a hydrogen atom or an alkyl group; each of R2, R3, R5, and R6 is a hydrocarbon group that may contain an ether bond; R2 and R3 may be bonded together to form a cyclic structure, and R5 and R6 may be bonded together to form a cyclic structure; AO represents an oxyalkylene group or oxyalkylene groups that may be the same or different; and n is a number of 1 to 1000 and represents an average addition molar number of the oxyalkylene group or oxyalkylene groups.