The present invention includes a first rotating operation unit which is arranged in one end section of an instrument main body such that a portion thereof protrudes from the top surface of the instrument main body, and switch-operated by the protruding portion being rotated by the thumb of a left hand, and a second rotating operation unit which is arranged posterior to the first rotating operation unit such that a portion thereof is exposed on a side surface side of the end section, and switch-operated by the exposed portion being rotated by a finger other than the thumb. Accordingly, when the left hand is placed on the end section, the thumb is placed corresponding to the first rotating operation unit, and the other finger is placed corresponding to the second rotating operation unit, whereby these rotating operation units can be rotated separately by only the left hand.