Abstract: An optical device selectively used in a variety of physical activities including leisure sports. The anti-fog optical device includes: an outer lens unit 10 having a front lens 14, an inner lens unit 20 engaged with the inner wall of the outer lens unit 10 to define an accommodation space 25 having a predetermined size between the outer lens unit 10 and the inner lens unit 20, a heat-blocking material disposed in the accommodation space 25 to block the transfer of heat, and an anti-leakage unit 30 engaged with a flange of the outer lens unit 10 to prevent the heat-blocking material from leaking. The outer lens unit 10 has an injection hole 16 allowing the heat-blocking material to be injected therethrough into the accommodation space 25. The anti-fog optical device prevents fog, whereby a clear field of view can be obtained for an activity.