A working machine for a board including a working device that selectively performs work for mounting conductive balls on a circuit board by a ball holder and 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 being 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.