PURPOSE: To provide the title form having no adverse effects on human health, causing no environmental pollution, excellent in reinforcing effect, affinity and mechanical strength, useful as a reinforcing material, etc., consisting mainly of a specific styrene-based polymer.

CONSTITUTION: The objective form 1-100mm in fiber length, 1-500μm in fiber diameter and ≥350dyne/cm in critical surface tension, consisting mainly of a highly syndiotactic styrene-based polymer with a degree of crystallinity of ≥30% and a birefringence of ≥20×10-3 in terms of absolute value. The other objective construction material comprising (A) 0.1-15wt.% of the present form and (B) a total of 85-99.9wt.% of calcium silicate, silicic stock and lime stock and/or inorganic fiber.