Accesory mounting structure for internal combustion engine

In an accessory mounting structure for an internal combustion engine, an accessory unit block 40 detachably supporting an engine accessory 10 is attached to the side 3f of the body 2 of the engine. The accessory unit block 40 has an upper portion 41 of a flat shape, and a lower portion 43 extending away from the engine body 2 to terminate in a water pump housing 43P. An upper fixing boss 41b is formed on an upper part of the upper portion 41, while a lower fixing boss 43b is formed on an end, remote from the engine body 2, of an upper-side edge 43U of the water pump housing 43P. The engine accessory 10 is fixedly mounted on the unit block 40 by attaching fixing brackets 11; 12L, 12R to the upper and lower fixing bosses 41b, 43b. The engine accessory 10 is thus compactly accommodated.