Bone-resorbing osteoclasts are differentiated from macrophages (MΦ) by M-CSF and RANKL. MΦ can be mainly classified into M1 and M2 MΦ, which are proinflammatory and anti-inflammatory, respectively, but little is known about their osteoclastogenic potential. Here, we investigated the osteoclastogenic potential of MΦ subtypes. When the two MΦ subtypes were differentiated into osteoclasts using M-CSF and RANKL, M2 MΦ more potently differentiated into osteoclasts than M1 MΦ. M2 MΦ generated with IL-4 or IL-10 also showed enhanced osteoclast differentiation compared with M1 MΦ induced by IFN-γ and lipopolysaccharide. In addition, robust bone-resorptive capacity and giant actin rings, which are features of mature osteoclasts, were observed in M2, but not M1 MΦ, under the osteoclast differentiation condition. Osteoclast differentiation was significantly increased in CD206 + M2 MΦ but not in CD86 + M1 MΦ. Compared with M1 MΦ, c-Fms and RANK were highly expressed in M2 MΦ. Enhanced osteoclastogenesis of M2 MΦ was mediated through sustained ERK activation, followed by efficient c-Fos and NFATc1 induction. Notably, the osteoclastogenic potential of M1 MΦ converted into M2 MΦ by exposure to M-CSF was higher than that of M2 MΦ converted into M1 MΦ by exposure to GM-CSF. Silencing IRF5, which is responsible for M1 MΦ polarization, increased osteoclast differentiation by enhancing c-Fms expression and activation of ERK, c-Fos, CREB, and NFATc1, which was inhibited by overexpression of IRF5. Collectively, M2 MΦ are suggested to be more efficient osteoclast precursors than M1 MΦ because of the attenuated expression of IRF5. K E Y W O R D S IRF5, M1 macrophage, M2 macrophage, osteoclast differentiation