The present invention addresses the problem of providing a method for measuring 25-hydroxy vitamin D and a measurement reagent, the method and the reagent being based on a competitive immunoassay. The present invention provides a reagent for measuring 25-hydroxy vitamin D based on a competitive immunoassay, which includes at least the following composition: (1) a Vitamin D derivative represented by the chemical formula (I) and/or (II), (2) an anti-25 hydroxy vitamin D antibody. In addition, the present invention provides a reagent for measuring vitamin D based on a competitive latex turbidimetric immunoassay (competitive LTIA) in which, in particular, a vitamin D derivative or an anti-25-hydroxy vitamin D antibody is immobilized on latex.