To obtain a new β-lactam useful for production of an N-substituted isoserine.

This β-lactam is represented by the formula (wherein R1 is OR6, SR7 or NR8R9; R2 is hydrogen, an alkyl, an alkenyl, an alkynyl, an aryl or a heteroaryl; R3 is an acyl; R4 is hydrogen, an alkyl, an alkenyl, an alkynyl, an aryl or a heteroaryl; R5 is COR10, or the like; R6 is hydrogen, an alkyl, an alkenyl; an alkynyl, an aryl, a heteroaryl or a hydroxy-protecting group; R7 is an alkyl, an alkenyl, an alkynyl, an aryl, a heteroaryl or a sulfhydryl-protecting group; R8 is hydrogen, an alkyl, an alkenyl, an alkynyl, an aryl or a heteroaryl; R9 is an amino-protecting group; R10 is an alkyl, an alkenyl, an alkynyl, an aryl or a heteroaryl).