A binocular includes two telescope systems with parallel axes, eyepiece frames, adjustable eye cups, operation controls placed between the telescope systems, a focus adjustment mechanism, a diopter correction mechanism, and a magnifying-power-varying mechanism. 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 frame of the rearmost lens of each eyepiece optical system includes an end surface facing a user that has a predetermined concave curvature. An eye cup locking mechanism is provided that securely locks each eye cup to a respective barrel of the binocular.