To supply the reagent, medium, and method for detection of Alicyclobacilus acidoterrestris, with which acidoterrestris can be rapidly detected and easily distinguished from acidocaldarius. After a sample including fruit juice is added to the medium for detection of Alicyclobacilus acidoterrestris containing chlor-2,3,5-triphenyl-2H-tetrazolium, acidoterrestris bacteria are detected based upon color change to red of the colony on the medium, which is kept at the temperature of 40 to 50° C.