To provide a special fiber structure degrading with time in a contact state with alkali such as freshly mixed concrete.

The fiber structure is obtained by using at least as a part a false twisted yarn comprising a polylactic acid-based fiber consisting essentially of a polylactic acid having ≥95% L-isomer, or a fiber obtained by adding an aliphatic polyester except the polylactic acid having alkali degradability to the polylactic acid-based fiber by copolymerization or blend, and having ≤12% shrinkage in boiling water and ≥1.76 cN/dtex tensile strength, and weaving or knitting the false twisted yarn. The fiber structure has a bag shape or a cylinder shape, and degrades in an alkali environment.