Ignition plug, control system, internal combustion engine, and internal combustion engine system

An ignition plug comprising:

• a tubular insulator having an axial hole extending in a direction of an axial line;
• a metallic shell disposed around an outer circumference of the insulator;
• a center electrode disposed in the axial hole of the insulator;
• and
• a ground electrode connected to a forward end of the metallic shell and facing the center electrode,
• wherein the metallic shell has a threaded portion to be engaged with a thread ridge of a mounting hole of an internal combustion engine, and
• a relational expression Ss/(Sa+Sb)≤2.6 is satisfied,
• where Ss is a surface area of an outer circumferential surface of the metallic shell extending from a rear end of the threaded portion to a forward end of the threaded portion,
• Sa is a surface area of that portion of the metallic shell which is to be exposed to combustion gas of the internal combustion engine;
• and
• Sb is a surface area of that portion of the insulator which is to be exposed to the combustion gas.