A manufacturing method of a semiconductor device may be provided. The method may include forming stacks including interlayer insulating layers and separated by a slit, the interlayer insulating layers surrounding a channel layer and stacked to be spaced apart from one another with an interlayer space interposed therebetween. The method may include forming a conductive pattern filling the interlayer space. The method may include forming an isolation layer on a surface of the conductive pattern by oxidizing a portion of the conductive pattern by performing an oxidizing process.