A cylinder block includes a support wall part that rotatably supports a crank shaft. The support wall part has a fitting recess part to which a bearing cap can be fitted. In each of the left and right corner parts where a bottom surface and a fitting surface of the fitting recess part intersect, a notch groove is formed that extends in the array direction of cylinder bores and has a substantially arc-shaped cross section when cut by the virtual plane along the extension direction thereof. As a result, stress having a greater value as the location becomes closer to the center part in the extension direction of the notch groove can be substantially uniform in the extension direction of said notch groove, and it is possible to effectively mitigate the stress concentration.