A working method using an AUV includes a step of working on a work object with a work device included in the AUV while causing the AUV to sail along the work object, a step of dropping and sinking a transponder to the bottom of water, a step of causing the AUV to sail toward a return destination, and a step of resuming work on the work object by causing the AUV to sail from a return destination to the vicinity of a work suspended position, at which work on the work object is suspended, based on information obtained by acoustic positioning using the transponder that is sunk to the bottom of water.