An operation unit of an image forming apparatus having a plurality of operation modes, allowing accurate and easy input by the user requesting switching of display to a screen image allowing selection of an operation mode, includes: a pilot lamp, a touch-panel display, and hardware keys including a power key, an energy-saving key and a home key. Two of the arms that are coupled to both sides of the third journal from the flywheel each have a center of gravity closer to its own counterweight compared to the centers of gravity of the arms that face the two arms with corresponding crankpins in between. If the tier status indicates no thread holds the nodal lock, the tier status is changed to indicate the nodal lock is owned, and if a global waiters and holder list is empty, an identifier of a node at which the thread executes is added to the global waiters and holder list.