This application discloses a cylinder block structure including: a cylinder block surrounding a cylinder array formed by a plurality of cylinders lined up in a first direction; a reinforcement plate including: a first fastening portion fastened to the cylinder block; and a second fastening portion fastened to the cylinder block at a position separated from the first fastening portion in a second direction that intersects with the first direction; and an oil pump that supplies oil to the cylinder block. The oil pump is fastened to the cylinder block together with the reinforcement plate. The reinforcement plate is interposed between the cylinder block and the oil pump, and has a length that is equal to or more than half of the cylinder array in the first direction.