An observation plug 102 includes a spark plug body 50, an objective optical system 10, and a set of light conduction paths 16. The spark plug body 50 ignites gas in a combustion chamber by an electric discharge in a discharge gap 60, and has an observation hole 4 penetrating in an axial direction at a location dislocated from the discharge gap 60. The objective optical system 10, which is provided in the observation hole 4 to be exposed into the combustion chamber, bends a course of light received from an incident surface 10a facing toward the discharge gap included in an observation area, and forms an image of the observation area within the observation hole 4. The set of light conduction paths 16 is provided in the observation hole 4, and divides the image of the observation area into a plurality of portions to be transmitted therethrough.