A combustion chamber structure for a spark-ignition internal combustion engine, the combustion chamber structure comprising: a combustion chamber configured to produce tumble flow that swirls in an axial direction of a cylinder; an ignition plug disposed in a central portion of an upper wall of the combustion chamber; and a guide portion that protrudes from the upper wall of the combustion chamber so as to surround the ignition plug, wherein the ignition plug is inserted and fitted in a plug hole formed at around the center of the guide portion, the guide portion extending to a larger extent at the intake port side as compared with at the exhaust port side, the guide portion being configured to guide airflow passing through the central portion of the combustion chamber in an intake-exhaust direction of the combustion chamber while dispersing the airflow around the guide portion, wherein the upper wall of the combustion chamber has two intake ports and two exhaust ports; the guide portion is shaped like a ginkgo leaf in a cross-section of a plan view as viewed from the piston side with an outer periphery that starts at a point between the two intake ports, extends along outer edges of valve seats of the respective intake ports, bends at points between the intake ports and the exhaust ports, to be directed to between the two exhaust ports, and ends at a point between the two exhaust ports; and a distance between an outer edge of the guide portion closer to the intake ports and a section line of the opening end of the plug hole is shorter than a distance between an outer edge of the guide portion closer to the exhaust ports and the section line in a cross-section of the guide portion passing through the center of the plug hole from the intake port side to the exhaust port side. a combustion chamber configured to produce tumble flow that swirls in an axial direction of a cylinder; an ignition plug disposed in a central portion of an upper wall of the combustion chamber; and a guide portion that protrudes from the upper wall of the combustion chamber so as to surround the ignition plug, wherein the ignition plug is inserted and fitted in a plug hole formed at around the center of the guide portion, the guide portion extending to a larger extent at the intake port side as compared with at the exhaust port side, the guide portion being configured to guide airflow passing through the central portion of the combustion chamber in an intake-exhaust direction of the combustion chamber while dispersing the airflow around the guide portion, wherein the upper wall of the combustion chamber has two intake ports and two exhaust ports; the guide portion is shaped like a ginkgo leaf in a cross-section of a plan view as viewed from the piston side with an outer periphery that starts at a point between the two intake ports, extends along outer edges of valve seats of the respective intake ports, bends at points between the intake ports and the exhaust ports, to be directed to between the two exhaust ports, and ends at a point between the two exhaust ports; and a distance between an outer edge of the guide portion closer to the intake ports and a section line of the opening end of the plug hole is shorter than a distance between an outer edge of the guide portion closer to the exhaust ports and the section line in a cross-section of the guide portion passing through the center of the plug hole from the intake port side to the exhaust port side.