To provide a crankcase of an internal combustion engine capable of reliably protecting a drain bolt and improving discharging property of an engine oil.

The crankcase 10 of an internal combustion engine 1 includes: a crank chamber 2 storing a crankshaft 5; and an oil pan chamber 3 communicating with the crank chamber 2 and formed below the crank chamber 2. The crankcase 10 has such a structure that a drain bolt hole 1 is formed to discharge oil from the oil pan chamber 3 and that a drain bolt guard 20 is disposed at a position adjoining to the drain bolt hole 15, the guard protruding outwardly from the crankcase to be higher than the head 31 of a drain bolt 30 attached to the drain bolt hole 15. Since the drain bolt guard 20 protrudes, at a position close to the drain bolt 30, outwardly from the crankcase to be higher than the head 31 of the drain bolt 30, the guard can prevent collision of an article to the drain bolt 30 and protects the drain bolt 30.