A pipe unit coupling structure includes a first member in which a first hole is formed, a second member including a pipe unit which is coupled to the first member by being inserted into the first hole, and a third member coupled to the second member by a coupling unit. The second member has an inclined surface formed at a tip of the pipe unit. The inclined surface is configured in such a manner that, when the inclined surface is pressed against the first member when the pipe unit of the second member is inserted into the first hole of the first member in a state where the second member and the third member are coupled to each other, a force is applied from the second member to the third member at the coupling unit, in a direction opposite to a direction of separation of the second member.