Type 2 diabetes is associated with sarcopenia. Resistance training and appropriate nutritional therapy are reported to be effective for muscle strength and mass. This study aimed to evaluate the effect of resistance training using elastic bands at home combined with a leucine-rich amino acid supplement on muscle strength, physical function, and muscle mass in elderly type 2 diabetes. We conducted a 48-week prospective single-center randomized controlled trial in 60 patients who were randomly allocated to one of three groups: control (C), resistance exercise (R), and resistance exercise plus supplement (RL). R and RL groups performed daily bodyweight resistance training with elastic bands exercises at home, and the RL group also took 6 g of a leucine-rich amino acid supplement daily. Knee extension strength (muscle strength), grip strength, usual gait speed (physical function), muscle mass, and cognitive function were assessed at 0 and 48 weeks. Although the change in knee extension strength from baseline was significantly increased by 6.4 Nm (95% CI 1.0, 11.7) in the RL group (p = 0.036), no significant difference was observed among the three groups (p = 0.090). Physical function, muscle mass, and cognitive function also had no changes during the study period among the three groups. No additive effect of a leucine-rich amino acid supplement on muscle strength or mass was observed. Although a post hoc analysis comparing with or without resistance training (C group vs. R + RL group) found that knee extension strength was significantly increased (p = 0.028), and cognitive decline was less (p = 0.046) than in the C group.