Disclosed is an optical gas sensor. The optical gas sensor includes: a light source which emits light; a gas collector which includes a cavity to be filled with gas to be sensed; an optical wave guider which guides light emitted from the light source to be output to the cavity, and guides the output light to be output again to the cavity after passing through the cavity of the gas collector; and an optical detector which detects light output from the optical wave guider. Thus, a path of light exposed to the gas to be sensed is increased within the cavity, thereby achieving miniaturization and sensing gas of low-concentration.