In a working machine for a circuit board including a working device that selectively performs work for mounting conductive balls on the circuit board by a ball holder 82 and performs work for transferring viscous fluid onto the circuit board by transfer pins, and a tray in which the viscous fluid is stored, when the conductive balls are to be mounted on the circuit board, the viscous fluid is transferred onto the circuit board by the transfer pins and the conductive balls having been immersed in the viscous fluid are mounted on the transferred viscous fluid. Accordingly, the conductive balls can be fixed onto the circuit board by the viscous fluid, which is transferred onto the circuit board by the transfer pins, and the viscous fluid that adheres to the conductive balls due to the immersion of the conductive balls in the viscous fluid. Therefore, the conductive balls can be appropriately mounted on the circuit board by a large amount of viscous fluid.