A threading device of a sewing machine is provided. The threading device includes a threading hook which moves forward to penetrate through an eye of a needle to capture a thread and which moves backward to insert the thread into the eye, a threading shaft which holds the threading hook at a lower end thereof, a threading movement input mechanism from which an operation for causing the threading hook to carry out a threading movement is input, an up-and-down member which moves, in accordance with the operation to the threading movement input mechanism, in up and down directions to rotate the threading shaft, a thread holding mechanism which holds the thread to be inserted into the needle, and a thread releasing mechanism which releases the thread that is held by the thread holding mechanism. The thread releasing mechanism releases the thread when the threading hook is moving backward.