A binocular includes two telescope systems with parallel axes, eyepiece frames, adjustable eye cups, operation controls positioned between the telescope systems, a focus adjustment mechanism, a diopter correction mechanism, and a magnifying-power-varying mechanism. The focus adjustment mechanism and diopter correction mechanism are arranged coaxially. The magnifying-power-varying mechanism includes rotatable cam rings and guide rings connected by a single connecting belt that moves perpendicular to the optical axes to rotate the cam rings. The connecting belt is supported and held in position so that it does not bind. The position of magnifying-power-varying lenses can be adjusted. During assembly, the magnifying-power-varying lenses can be mounted in the guide rings in a number of positions by rotating the lens frames holding the lenses about the optical axis and inserting sliders in linear guide grooves. Further, after assembly, the position of the magnifying-power-varying lenses along the optical axes in relation to the guide rings and cam rings can be adjusted.