A spark plug for a gas engine includes a ground electrode body and a center electrode body. A ground electrode of the ground electrode body is welded on a surface of a ground electrode base material opposing to the center electrode body. The ground electrode protrudes toward the center electrode body. The ground electrode is made of a material containing iridium or platinum as a main component. The ground electrode base material is made of a heat-resistant nickel alloy. The ground electrode is impacted in a recess of the ground electrode base material. The spark plug satisfies a condition 0.3≦h/H≦0.5 when H represents the thickness of the ground electrode base material in an intrusion direction of the ground electrode, and h represents an intrusion length of the ground electrode that defines a length of the ground electrode impacted in the ground electrode base material in the intrusion direction.