To provide a ball valve type underground fire hydrant capable of surely draining residual water remaining in the fire hydrant through a drainage valve and meeting the clog thereof with partial repair even when the fire hydrant is chocked up with dust or the like.

An interlocking mechanism 25 interlocking with opening and closing of a ball-shaped valve disc to automatically open and close a drainage valve 23 is provided between a rotary shaft 17 of the ball-shaped valve disc and drainage valve 23 connected to a drain outlet of a valve box 12. The interlocking mechanism 25 is so set that the drainage valve 23 simultaneously opens when the ball-shaped valve disc is closed, and residual water remaining in a fire hydrant 11 is automatically drained out through the drainage valve 23.