A manufacturing method of a semiconductor device, includes providing a substrate; forming a stacked gate, including a floating gate and a control gate, on the substrate; forming a stacked gate by a deposition of a select gate conductive layer on the stacked gate; forming a trench in the stacked gate by etching the stacked gate to separate a first select gate pattern and a second select gate pattern; and forming a first select gate, a second select gate, a first transistor, and a second transistor simultaneously through an etch-back process of the stacked gate.