A 3D observation device with glassless mode, where an observation hole in which two polarization plates having different polarization directions are arranged left/rightward and a monitor on which an image for a left eye and an image for a right eye are connected to form a 3D image box. The vertical rotation axis are provided on both left and right surfaces of the 3D image box and the vertical rotation bars that are linked with the rotation axis, and a support bar is provided on a lower part of the vertical rotation bar. Further, a viewer can view separately with glassless mode an image for a left eye through a left polarization plate of the observation hole and an image for a right eye through a right polarization plate of the observation hole in which the image for a left eye and the image for a right eye are overlapped.