Provided are a polymethylpentene conjugate fiber, which is capable of imparting to a lightweight polymethylpentene fiber an ability to develop a vivid and deep color, and a porous polymethylpentene fiber, which has a lightweight, a high pore diameter uniformity and a high porosity retention ratio against an external force, said polymethylpentene conjugate fiber and said porous polymethylpentene fiber being appropriately usable as a fiber structure for woven knitted goods, non-woven fabrics, yarns, cotton waddings, etc. The polymethylpentene conjugate fiber is characterized by having an island-in-sea structure wherein the sea component comprises a polymethylpentene-based resin and the island component comprises a thermoplastic resin. The porous polymethylpentene fiber, which comprises a polymethylpentene-based resin, is characterized in that the coefficient of variation (CV) of pore diameter at the fiber cross section is 1-50%.