To improve stability of coupling between an auxiliary device driven by a camshaft and a cylinder head in an internal combustion engine.

An auxiliary device mounting structure for an engine 1 includes: a cylinder head 2 having a valve chamber 16; an exhaust-side camshaft 37 having an extension 51 at a terminal portion; a lower housing 42 coupled to a coupling surface 17 that is an end surface of the cylinder head on the side of the valve chamber, and protruding outward beyond a left edge wall 13; an upper housing 43 coupled to the lower housing and defining a housing chamber 48 receiving the extension between the lower housing and the upper housing; and a plunger 86 entering the housing chamber. The auxiliary device mounting structure also includes: a fuel injection pump 4 supported by a second housing; an auxiliary device drive cam 61 which is formed in the extension for moving the plunger back and forth; and a first bearing lower part 53 and a first cam cap 54 which are provided integrally in the lower housing for supporting the extension portion in a rotatable manner.