To provide a bearing unit for supporting a wheel capable of preventing intrusion of foreign matters such as muddy water into an internal space while constantly keeping the internal space of a bearing under a pressure equal to an atmospheric pressure.

A cap 5a has an outer plate portion 21 and an inner plate portion 22, and a labyrinth space 26 is formed between both plate portions 21 and 22. An external air hole 23 communicating between an external space and the labyrinth space 26 is formed near a vertical central portion of the outer plate portion 21, an internal air hole 24 communicating between the labyrinth space 26 and an internal space 11 is formed at a vertical upper portion of the inner plate portion 22, and a discharge hole 25 communicating between the external space and the labyrinth space 26 is formed at a vertical lower portion of the outer plate portion 21. In this state, the internal space 11 and the external space are communicated through the external air hole 23, the labyrinth space 26, and the internal air hole 24.