A cooling structure of an internal combustion engine includes a cylinder block having a plurality of cylinder bores; a cylinder head disposed on an upper portion of the cylinder block; two camshafts disposed on the cylinder head so as to be juxtaposed to each other in parallel to a crankshaft; camshaft housing chambers formed on the cylinder head to house the respective camshafts; a recessed portion provided between the camshaft housing chambers; a plug seat formed between the camshafts and in the recessed portion; a cooling air passage formed between the cylinder bores; and cooling air introduction passages communicating from the cooling air passage to the recessed portion of the cylinder head.