To inexpensively compose an adjusting device for a brake adjuster with a simple structure by utilizing an existing rotary tool.

A nut adjusting device 10 is composed of the rotary tool 11, a rotary shaft 12, and a clutch 13 in order to adjust a feed-in amount of an adjusting nut in the brake adjuster for a two-wheeled motor vehicle. The clutch 13 has a tubular socket member 14. The adjusting nut 7 is fitted to a socket 20 provided to the end part of the socket member 14. A screw rod 6 is inserted to a shaft hole 21 and rotated with the rotary tool 11. When the adjusting nut 7 reaches a prescribed feed-in amount to the screw rod 6, the clutch is disconnected by pushing a push rod 16 after the abutment of one end of the screw rod 6 to the end part of the push rod 16. Consequently, the transmission of rotational driving force from the rotary tool 11 is cut off.