A plastic cap, including: a cap body which is composed of a top plate section and a skirt section, has a pouring nozzle formed on an outer surface of the top plate section so as to surround an opening-scheduled portion, and is configured to be fitted and fixed to a container mouth; and a top lid composed of a top face, and a circumferential wall suspending from an outer peripheral edge of the top face, wherein the top lid has, formed therein, a protrusion protruding downward from an inner surface of the top face and having an engaging portion on an outer surface thereof; and the cap body has an inner plug (30, 40) formed therein, the inner plug (30, 40) is composed of an annular sidewall (31, 41) having an outer diameter enabling the annular sidewall (31, 41) to intimately contact an inner surface of the pouring nozzle, and a bottom (32, 42) formed at a lower part of the annular sidewall (31, 41), the inner plug (30, 40) is formed integrally with the cap body via a breakable weakened portion at a position below a seal portion of the pouring nozzle having an inner diameter enabling the seal portion to intimately contact the inner plug (30, 40), and an engaged portion (33, 43) vertically engageable with the engaging portion of the protrusion is formed on an inner surface of the annular sidewall (31, 41), characterized in that: with the top lid being fixed to the cap body, the breakable weakened portion is configured to be broken, the engaged portion (33, 43) of the inner plug (30, 40) released from the cap body is configured to climb over the engaging portion of the protrusion, whereupon the engaged portion (33, 43) and the engaging portion become engageable with each other, and an outer surface of the annular sidewall (31, 41) and the inner surface of the pouring nozzle are configured to make intimate contact. a cap body which is composed of a top plate section and a skirt section, has a pouring nozzle formed on an outer surface of the top plate section so as to surround an opening-scheduled portion, and is configured to be fitted and fixed to a container mouth; and a top lid composed of a top face, and a circumferential wall suspending from an outer peripheral edge of the top face, wherein the top lid has, formed therein, a protrusion protruding downward from an inner surface of the top face and having an engaging portion on an outer surface thereof; and the cap body has an inner plug (30, 40) formed therein, the inner plug (30, 40) is composed of an annular sidewall (31, 41) having an outer diameter enabling the annular sidewall (31, 41) to intimately contact an inner surface of the pouring nozzle, and a bottom (32, 42) formed at a lower part of the annular sidewall (31, 41), the inner plug (30, 40) is formed integrally with the cap body via a breakable weakened portion at a position below a seal portion of the pouring nozzle having an inner diameter enabling the seal portion to intimately contact the inner plug (30, 40), and an engaged portion (33, 43) vertically engageable with the engaging portion of the protrusion is formed on an inner surface of the annular sidewall (31, 41), characterized in that: with the top lid being fixed to the cap body, the breakable weakened portion is configured to be broken, the engaged portion (33, 43) of the inner plug (30, 40) released from the cap body is configured to climb over the engaging portion of the protrusion, whereupon the engaged portion (33, 43) and the engaging portion become engageable with each other, and an outer surface of the annular sidewall (31, 41) and the inner surface of the pouring nozzle are configured to make intimate contact. with the top lid being fixed to the cap body, the breakable weakened portion is configured to be broken, the engaged portion (33, 43) of the inner plug (30, 40) released from the cap body is configured to climb over the engaging portion of the protrusion, whereupon the engaged portion (33, 43) and the engaging portion become engageable with each other, and an outer surface of the annular sidewall (31, 41) and the inner surface of the pouring nozzle are configured to make intimate contact.