A frame for a serially connected axial flow fan unit includes a first housing and a second housing coupled together. The first housing includes a first axial locking mechanism and a first unlocking mechanism, and the second housing includes a second axial locking mechanism and a second unlocking mechanism. The first and the second axial locking mechanism are locked against each other through their movement along their axis. The first and the second unlocking mechanism unlock the first and the second housing with a twisting force equal to or greater than a predetermined value when the first and the second housing are twisted relative to each other. When the first and the second end portion are brought into contact, the first and the second axial locking mechanism are locked against each other and the first and the second unlocking mechanism engage with each other.