The present invention has an object of providing a novel multilayer composite fiber using polyacetal mainly derived from methanol, which is a petroleum-independent raw material, and having a low environmental load, and also providing a nonwoven fabric obtained by thermally bonding such fibers. According to the present invention, a multilayer composite fiber comprising a resin containing polylactic acid as a main component and a resin containing polyacetal as a main component, in which the components both form continuous layers in an axial direction of the fiber, and also a nonwoven fabric obtained by thermally bonding such fibers, can be provided.