A threading device of a sewing machine is provided. The threading device includes a threading hook which moves forward to enter an eye of a needle to catch a needle thread, and moves rearward with the needle thread being caught to insert the needle thread through the eye, a threading shaft which holds the threading hook, an operating member which is operated to move the threading shaft, a threading operation mechanism which transmits the movement of the threading shaft to the threading hook to move the threading hook back and forth, and a restricting device which makes the threading operation mechanism inoperable when the spreader mechanism is attached to the sewing machine main body, and makes the threading operation mechanism operable when the spreader mechanism is detached from the sewing machine main body.