Power transmission device

The invention provides a power transmission device, wherein a rotation radius could be detected very accurately without use of a high-sensitivity sensor. The power transmission device comprises a control part and a crank-link mechanism which can convert rotation motion of an input shaft to sway motion of a sway part, wherein the control part changes a rotation radius of a rotation part so as to change a gear ratio. The control part comprises a drive source rotation speed detection part, an orthogonal function generation part, a phase detection part and a related constituent detection part, wherein the drive source rotation speed detection part detects a rotation speed of a prime motor; the orthogonal function generation part generates an orthogonal function synchronous with the rotation speed according to the rotation speed of the prime motor; the phase detection part detects a phase of the sway part which sways synchronously with drive force of the prime motor; the related constituent detection part applies