To provide a fibrillated fiber suitable for improving mechanical strength in the thickness direction of wet friction material.

The fibrillated fiber aggregate has fibrils that are composed of an organic high-molecular polymer and has a fibril diameter of 1 m or less. The fibrillated fiber aggregate satisfies the following requirements of (a) among fibers constituting the fiber aggregate, the content of crimped fibers has one or more pairs of crimped ridges and valleys in main fibers represented by the following formula: the crimped fiber content =Nc/N0.5100 is 20% or more (wherein Nc is the number of crimped fibers having one or more pairs of crimped ridges and valleys in the main stem fibers; N0.5 is the number of total fibers having a fiber length of 0.5 mm or more), and (b) a specific surface area of fiber aggregate is 3-20 m2/g.