A level includes a plate secured to a casing and rotatable relative to the casing between a closed and an open positions. The display unit is configured to display a medical image on which a plurality of annotations are superimposed. A surrounding cover deformable only in accordance with the motion of upward extension and downward contraction of the threaded cylinder and the center threaded rod is interposed between the upper end part of the center threaded rod and the case body. Other embodiments are also disclosed. Each cell also has a put component part configured to operate according to the sender time domain including a put token passing circuit and put controller circuit.