A container closure of synthetic resin to be applied to a container having an external thread and an engaging jaw portion positioned below the external thread formed on the outer periphery of the mouth-and-neck portion, and the container closure of synthetic resin comprises: a body having a top panel wall and a cylindrical skirt wall extending perpendicularly downward from the peripheral edge of the top panel wall, and an internal thread disposed on the inner periphery of the skirt wall so as to be screwed to the external thread, and; a cylindrical tamper evident skirt portion connected to the skirt wall of the body via a plurality of breakable bridge portions disposed circumferentially with spacing, the tamper evident skirt portion has an engaging means disposed on the inner periphery of the tamper evident skirt portion so as to engage with the engaging jaw portion; the engaging means is formed of: either a plurality of engaging pieces (24a-24i) disposed circumferentially with spacing and extending upward and radially inward from the base end connected to the tamper evident skirt portion, or an annular engaging piece extending in a circumferentially continuous manner and extending upward and radially inward from the base end connected to the tamper evident skirt portion, and a protruding piece that protrudes downward is disposed at the skirt wall of the body, and unbroken connecting pieces (38a, 38b) that connect the skirt wall of the body to the tamper evident skirt portion and that allow the body to ascend relative to the tamper evident skirt portion are disposed, wherein extending portions that extend upward are formed at the tip ends of the engaging pieces (24a-24i) in at least an angle region in which the protruding piece is present, and the extending portions formed in the angle region in which the protruding piece is present are configured to be positioned between the engaging jaw portion of the mouth-and-neck portion and the protruding piece in a state where the container closure of synthetic resin is mounted on the mouth-and-neck portion, characterized in that the radially outer side face of each of the extending portions gradually protrudes radially outward when viewed from below and then gradually recedes radially inward when viewed from below. a body having a top panel wall and a cylindrical skirt wall extending perpendicularly downward from the peripheral edge of the top panel wall, and an internal thread disposed on the inner periphery of the skirt wall so as to be screwed to the external thread, and; a cylindrical tamper evident skirt portion connected to the skirt wall of the body via a plurality of breakable bridge portions disposed circumferentially with spacing, the tamper evident skirt portion has an engaging means disposed on the inner periphery of the tamper evident skirt portion so as to engage with the engaging jaw portion; the engaging means is formed of: either a plurality of engaging pieces (24a-24i) disposed circumferentially with spacing and extending upward and radially inward from the base end connected to the tamper evident skirt portion, or an annular engaging piece extending in a circumferentially continuous manner and extending upward and radially inward from the base end connected to the tamper evident skirt portion, and a protruding piece that protrudes downward is disposed at the skirt wall of the body, and unbroken connecting pieces (38a, 38b) that connect the skirt wall of the body to the tamper evident skirt portion and that allow the body to ascend relative to the tamper evident skirt portion are disposed, wherein extending portions that extend upward are formed at the tip ends of the engaging pieces (24a-24i) in at least an angle region in which the protruding piece is present, and the extending portions formed in the angle region in which the protruding piece is present are configured to be positioned between the engaging jaw portion of the mouth-and-neck portion and the protruding piece in a state where the container closure of synthetic resin is mounted on the mouth-and-neck portion, characterized in that the radially outer side face of each of the extending portions gradually protrudes radially outward when viewed from below and then gradually recedes radially inward when viewed from below.