To provide a block dividing structure for dividing a cylinder block and collecting strength requiring components while minimizing high quality components as required and giving less distortion to cylinder portions when assembled.

In the block dividing structure for an engine having a bore part 10, a crank case part 13a, and a crank cap 17, a crank cap bolt 18b is fastened to a first female screw portion 21b of a thrust member 21 and the crank cap 17 is thus fitted to the crank case part 13a across the bore part 10. The thrust member 21 thrusts the bore part 10, and a cylinder head bolt 18a is fastened to a second female screw portion 21a of the thrust member 21, and a cylinder head 16 is thus fitted thereto. When the bore part 10 is fitted to the crank case part 13a, an outer wall part 13b encircling the bore part 10 is integrally formed on the crank case part 13a.