A rocker arm installed module 1 is constructed by installing the cam shaft 3, the rocker arm 4 and the pivot 5 to the housing 2. The housing 2 is provided with a drop-off preventing means 21 for supporting and preventing the rocker arm 4, which is disposed on the pivot 5, from dropping off. The rocker arm installed module 1 is constructed by disposing the rocker arm 4 among the cam 31 of the cam shaft 3, the pivot 5 and the drop-off preventing means 21 in the housing 2 to form a module, the module being configured to be disposed in a concave portion 61 of the cylinder head 6 while preventing the rocker arm 4 from dropping off.