To provide a thin and light fiber base material, a fiber-reinforced composite material, a method for producing the fiber base material, and a method for producing the fiber-reinforced composite material.

A fiber base material 20 includes carbon fibers having a fiber diameter of 10 nm to 1000 nm and being made into a form of a nonwoven fabric, and has an average fiber length per one square meter of 100 mm or more. The method for producing the fiber base material 20 includes: an adhering step of spraying a solution of a material for a carbon fiber on a surface of a mandrel 33 from a discharge nozzle of an electrospinning apparatus used in an electrospinning method to make nanofibers having a fiber diameter of 10 nm to 1000 nm adhere to the surface of the mandrel 33; and a carbonizing step for carbonizing the nanofibers to form carbon fibers.