To provide a power transmission device of a simple structure capable of easily setting breakaway torque and having a high degree of freedom in designing.

A spherical projection 18 is formed on a driving-side rotating member 11, a through hole 21D engaged with the spherical projection 18 is formed in a driven-side rotating member 12, and an engagement mechanism of the spherical projection 18 and the through hole 21D is released against the energizing force of an elastic member, when an overload is applied between the driving-side rotating member 11 and the driven-side rotating member 12. Whereby a rubber member used in a conventional device becomes unnecessary. The elastic member is not mounted between the spherical projection 18 and the through hole 21D configurating the engagement mechanism, the mounting position of the elastic member is not restricted, which increases a degree of freedom in designing, and further sufficient vibration-proof performance can be achieved.