The present invention provides a method and a reagent for measuring periostin contained in a sample with improved accuracy, a method for improving accuracy in measurement of periostin, and a method of testing for pulmonary fibrosis or interstitial pneumonia with improved accuracy. The antibody of the present invention binds to at least one region selected from the group consisting of an EMI region, an R1 region, an R2 region, and an R3 region of periostin or a cleavage product thereof. The method and the reagent for measuring periostin and the method for improving accuracy in periostin measurement of the present invention is characterized by detecting at least one region selected from the group consisting of an EMI region, an R1 region, an R2 region, and an R3 region of periostin. The method of testing for pulmonary fibrosis or interstitial pneumonia of the present invention includes the steps of a) measuring the amount or concentration of periostin in a sample derived from a subject, which measuring includes detecting at least one region selected from the group consisting of an EMI region, an R1 region, an R2 region, and an R3 region of periostin and b) comparing the amount or concentration of the periostin in the sample derived from the subject with an amount or concentration of periostin in a sample derived from a living body not suffering from pulmonary fibrosis and interstitial pneumonia.