A method of manufacturing a semiconductor device according to an embodiment of the present disclosure may include forming a first sacrificial layer including a first portion and a second portion having a thickness thicker than a thickness of the first portion, forming a stack including first material layers and second material layers alternating with each other on the first sacrificial layer, forming a channel structure passing through the stack and extending to the first portion, forming a slit passing through the stack and extending to the second portion, removing the first sacrificial layer through the slit to form a first opening, and forming a second source layer connected to the channel structure in the first opening.