PURPOSE: To obtain a binocular adapter which can be constituted by combining monocular telescopes and selectively take advantage of both of the monocle and binoculars.

CONSTITUTION: Arm-like holding parts 14A and 14B to which two telescopes 17 and 18 are attached and which hold them are freely turnably provided on the supporting shaft 12 of a support 11. Moreover, the holding parts 14A and 14B are turned in opposite directions by sliding a pin 23 connected to linkage parts 21 and 22 in a long groove 24. Moreover, an idler gear 27 is arranged on the supporting shaft 12 and the focusing rings 30A and 30B of both telescopes 17 and 18 are meshed with the idler gear 27. Thus, two telescopes 17 and 18 are used as the binoculars and the eye width of the binoculars can be adjusted by the movement of the telescopes 17 and 18. Further, both focusing rings 30A and 30B are linked in the same direction by the idler gear 27, to easily focus the binoculars.