The present invention relates to an optical device for both a dot sight mode and a scope mode, the optical device including: a dot-sight main body which is formed with a dot sight region where a light source and a reflective mirror are installed to display light emitted from the light source as a dot on an external target, and operates in the dot sight mode; and a scope structure which includes an objective lens and an eyepiece lens to magnify and see the external target, and is rotatable inside the dot-sight main body, the scope mode being selected when the scope structure is rotated and inserted in the dot sight region. With this, there is provided an optical device for both a dot sight mode and a scope mode, in which a scope structure is installed to be retractable inside a dot sight device and thus both the dot sight mode and the scope mode are supported, thereby aiming at an external target through quick mode switching, and there is no need for an additional scope device.