Disclosed is a cap unit wherein safety is increased by preventing a reversed flow of a puncture-sealing agent. A cap main body (33) comprises an air flow passage (31) consisting of a vertical flow passage (31A) extending downward from an air passage upper opening (31a), a lateral flow passage (31B) laterally extending from an air intake port (31b), and a connecting flow passage (31C) connecting therebetween. The air flow passage (31) is provided with a reversed flow preventing device (22). The cap main body (33) is provided with a vertical hole (40) consisting of first to fourth vertical holes (40A to 40D) which have inner diameters increasing downwardly from the upper end in a stepwise manner, and the first to third vertical holes (40A to 40C) form the vertical flow passage (31A).