Pompe disease is an autosomal recessive myopathic disorder caused by the deficiency of lysosomal acid α-glucosidase (GAA). Recently, we showed that function of mutant GAA in fibroblasts derived from Pompe disease patient carrying c.546G>T mutation is improved by treatment with proteasome inhibitor bortezomib as well as pharmacological chaperone (PC). However, bortezomib-responsive GAA mutations are not fully characterized. In this study, we showed the effect of bortezomib on different mutants of GAA in patient fibroblasts and transiently expressed HEK293T cells. Bortezomib increased the maturation and residual activity of GAA in patient fibroblasts carrying PC-responsive M519V and PC-unresponsive C647W mutations. Enhanced colocalization of GAA with lysosomal marker LAMP2 was also observed in patient fibroblasts after treatment with bortezomib. When four distinct mutant GAAs, which show different response to PC, were overexpressed in HEK293T cells, bortezomib improved the activity of M519V, S529V, and C647W in them (1.3-5.9-fold). These results indicate that bortezomib enhances the activity of some PC-unresponsive GAA mutants as well as PC-responsive mutants.