The effect of electrochemical reduction on the single-molecule conductance of fullerene C-60 derivatives was studied by scanning tunneling microscopy. Three types of C-60 derivatives were synthesized, a monoadduct having an amino-terminated linker and two bisadducts having two linkers at different positions (trans2 and trans3). Each C-60 derivative was immobilized on a gold surface by an amino-gold linkage, confirmed by infrared reflection-absorption spectroscopy. The immobilized C-60 derivatives showed reversible and multiple reduction peaks in the cyclic voltammogram in dimethylformamide (DMF) at almost the same potentials as those in solution, showing the redox properties of the molecules are intact on gold. Single-molecule conductances of the bisadducts, which can span between a scanning tunneling microscopy (STM) tip made of gold and substrate with the two linkers, were determined by the STM break-junction measurements in water and DMF. The conductances were 6.1 +/- 4.5 nS in water and 4.9 +/- 1.7 nS in DMF for the trans2 bisadduct and 8.4 +/- 3.4 nS in water and 7.9 nS +/- 2.8 in DMF for the trans3 bisadduct. By using a potential-controlled STM setup, the tunneling current through a single molecule was recorded with sweeping the potentials of the tip and substrate. The trans2 bisadduct showed significant changes in the current when the reductions of the C-60 moiety occur. Some current curves showed multiple peaks, and the other curves showed stepwise increase and decrease at the C-60 reduction and subsequent reoxidation. Statistical analysis afforded stepwise switching of the conductance as the average behavior and suggested that the electron tunneling through the C-60 derivative is enhanced as it accepts electrons.