As well as to adjust the stereoscopic effect of the stereo image, the stereo microscope with a three-dimensional stereo image of an accurate focus even if the control is started. The stereo microscopes are placed on top of the focusing lens, and is placed on top of a pair of zoom unit, each of the zoom unit are spaced apart from each other a distance adjusted to adjust the stereoscopic effect of the stereo image in conjunction with each of the zoom unit is sikidoe by refractive receiving a beam from the pair of the first prism and a pair of the second prism and the second prism for refracting can redirect receive the beam from the first prism unit is moved reverses direction, the each of the first prism is a spaced distance from the first prism and a second prism of the distance of the variation beam path pair between the second prism adjusted by being moved in cooperation above and each of the zoom unit wherein by compensating for the variation beam path distance between the first prism and the second prism agent so as to maintain a constant beam path 3 It includes a prism.