To provide a polymer alloy chip constituted of sea-island structure capable of stably producing superextrafine polyamide fibers in the form ranging from a microfiber to a nanofiber with a length of at least 0.2-200 mm in the longitudinal direction, and to provide methods for producing a polymer alloy fiber from the chip and the superextrafine polyamide fiber, respectively.

The polymer alloy fiber 4 of sea-island structure is composed of at least two component polymers. In the polymer alloy fiber 4, the sea component 3 is based on a stereocomplex polylactic acid, while the island component 2 is based on a polyamide; wherein the polymer viscosity of the island component 2 is 50-450 Pa. s higher than that of the sea component 3 at 190-260C, the average fiber diameter of the island component 2 is 0.1-20 m, and the fiber diameter variation of the island component 2 is 5-35%. The superextrafine polyamide fiber is obtained by subjecting the polymer alloy fiber 4 to alkali dissolution. The method for producing the superextrafine polyamide fiber is also provided.