1. A 3D observation device with glassless mode, comprising: a 3D image box provided with a screen on which an image for a left eye and an image for a right eye of a 3D image are displayed doubly; an observation hole provided left/rightward with two polarization plates having different polarization directions and arranged on a front surface of the 3D image box; a vertical rotation axis that is provided on both left and right surfaces of the 3D image box to rotate vertically the 3D image box; and a support bar that is provided on a lower part of the vertical rotation axis and rotates simultaneously the 3D image box and the vertical rotation axis ( 7 ) wherein a viewer views 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 among the 3D image in which the image for a left eye and the image for a right eye are overlapped.