To provide a synthetic resin cap capable of preventing fraudulent opening without changing the size of the cap or fitting force to a container, and allowing a consumer to easily determine presence of opening.

In this synthetic resin cap 1 including an inside plug 3, an upper lid 2 to be locked to the inside plug 3 by threaded engagement, and a sealing strip 4 integrally formed on the lower end face 2a of the upper lid 2 through a bridge 14, an engagement projecting part 13 is formed on the inner surface of the sealing strip 4, and a recessed part 11 continuing to the lock projecting part and a side face lower side on a lid closing direction side of the lock projecting part is formed on an outer surface of a sidewall 2B of the inside plug 3.