A locking mechanism (6) of the robot arm coupling device (1) includes: a concave engagement portion (3a) on the master plate (3); convex engagement portion (4a) on the tool plate (4) for inserting into the concave engagement portion (3a); a plurality of steel balls (16) installed at an external circumferential wall portion (15) of the concave engagement portion (3a), and are capable of being changed over between locking positions and unlocking positions; and an annular fluid pressure cylinder (20) capable of changing over the positions of the plurality of steel balls (16); and a space is defined more radially inward than the plurality of steel balls (16) and the fluid pressure cylinder (20).