PURPOSE: To readily detect bacteriophage in a low cost with a low concentration by bringing sample water containing bacteriophage into contact with clinoptilolite or sepiolite.

CONSTITUTION: A powdery adsorbent (A) of clinoptilolite as a species of zeolite composed of composition expressed by formula I or sepiolite as natural ore composed of composition expressed by formula II is added to a sample water containing phage (e.g., Bacillus natto phage) and brought into contact by mixing at about 25°C for about 24hr, then filtered to obtain supernatant (B). Next, said component B and sample water are cultured in a medium (C) composed of LB medium containing bactotryptophan, etc., in which Bacillus natto is previously cultured, then number of phage granules, thus phage-adsorbed amount of the component A is determined from difference of phage granule numbers between sample water and the component B. Next, the component A adsorbed phage is washed in aseptic water and resultant washed solution and powder of the component A are cultured in LB medium to detect bacteriophage without deactivation, without generating spot of dissolved bacteria and generating spot of dissolved bacteria in surrounding of powder of the component A.