PURPOSE: To easily mould a main body having a load sieve storage space by die-casting and to easily install a load sieve in the load sieve storage space while effectively preventing chain roll-in by way of providing a chain roll-in preventive part on a chain guide body and without interfering with the chain roll-in preventive part.

CONSTITUTION: By providing an opening part 6 to continuously open a load sieve storage space 7 downward with the load sieve storage space 7 on a main body 1 furnished with the load sieve storage space 7 and engaging a chain guide body 8 having a chain roll-in preventive part 8a with a central part of this opening part 6, it is possible to easily mould the main body 1 integrally by aluminium die-casting, and by building in the chain guide body 8, it is possible to form a chain conducting passage of the chain guide body 8 and to effectively prevent roll-in of a chain.