To provide a two cycle combustion engine utilizing a connecting structure of a shape that can be formed inexpensively with a simple process to enable the efficiency of cooling of the cylinder block to be maintained at a high level and, also, that can firmly connect the cylinder block with the crankcase with fastening forces exerted by the fastening bolts, the two cycle combustion engine includes a crankcase 2 including first and second crank casings 2A and 2B that are split in a direction conforming to a crank axis C for operatively supporting a crankshaft 11, and a cylinder block 1 having a cylinder bore 1a defined therein and fixedly mounted atop the crankcase 2 by means of a plurality of fastening members 38 and 39 fitted in a direction upwardly from a crankcase-side.