The invention relates to an eccentric oscillating-typr planetary gear device. Elastic deformation of bridge portions in an externally toothed gear and external teeth is suppressed, thereby extending the life of tooth surfaces of external teeth 19, improving vibration characteristics, and markedly increassing output torque while preventing a planetary gear device 11 from becoming large in size. Toachieve the above, a ratio obtained by dividing the diameter D of each of pins constituting internal teeth 14 by the pitch P of the internal teeth 14 is made smaller to an extent where tooth tops 19aof the external teeth 19 are radially outside the inner periphery 15a of an internally toothed gear 15. Alternatively, a meeting point C where action lines S of reaction force K of drive force components meet is moved more radially outward compared with a known eccentric oscillating-type planetary gear device so as to be positioned between a pin circle P passing the centers of all the internal teeth (pins) 14 and an outer end passing circle G passing radially outer ends of through holes 22. In addition, the amount H of eccentricity of an externally toothed gear 18 relative to an internally toothed gear is set not less than 0.5 times the radius R of the internal teeth (pins) 14.