PURPOSE: To provide extremely lightweight porous polypropylene fiber, excellent in heat insulating properties and having a large pore diameter and a high porosity and a method for producing the above-mentioned fiber.

CONSTITUTION: Polypropylene having 0.1-10 MI value is melt spun, heat-treated at 100-165°C temperature for ≥30min, then cold drawn and subsequently hot drawn at 120-165°C temperature. In the process, the deformation rate in the hot drawing is regulated to ≤10% per sec and the total draw ratio is regulated to 600-2000%. Thereby, porous polypropylene fiber in which micropores in a rectangular form, surrounded and formed from microfibrils oriented in the fiber longitudinal direction and knot parts composed of stacked lamellas communicate from the fiber surface to the central part over the whole is produced. The aforementioned fiber has 1-10μm average pore diameter of the micropores and 70-95% porosity measured with a mercury porosimeter.