A spark plug for an internal combustion engine includes a tubular housing, a tubular insulator, a center electrode, a ground electrode and a guide member. The guide member is configured to guide the flow of an air-fuel mixture in a combustion chamber of the engine to a spark gap formed between the center electrode and the ground electrode. Moreover, in the spark plug, the following dimensional relationships are satisfied: b≧−67.8×(a/D)+27.4; b≦−123.7×(a/D)+64.5; −0.4≦(a/D)≦0.4; and 0°≦b≦90°. Further, with an oblique angle θ being in the range of 0 to 30°, the following dimensional relationship is also satisfied: 0.8≦r/R≦1. Consequently, the spark plug can secure, with a simple configuration, a stable ignition capability regardless of the mounting posture of the spark plug to the engine.