A speed adjusting device of a speed changer being used in a middle speed changer and a rear speed changer of a bicycle, and contains: a base including a first hole fixed on a central portion thereof to insert a first central spindle, and a holder and a fitting member both of which extend from a front end of the base to connect with an adjusting loop, and the fitting member including a ring member fitted thereon; an adjusting loop including a plurality of first screw bores to lock a front cover and a second hole disposed on a central position thereof to receive the first ratchet, the first actuating lever, the ring member, and the hook, the second hole including a groove fixed at a suitable position of an outer side thereof; a front cover including a first orifice formed on a central portion thereof to insert a first screw.