A forward primer of a primer set in accordance with the present invention for detecting BLV is a mixture of (1) a first primer consisting of a polynucleotide including 15 or more successive bases including the 16th C in the base sequence of SEQ ID NO: 1 and (2) a second primer consisting of a plurality of kinds of polynucleotides including at least the first to 15th bases in the base sequence of SEQ ID NO: 2. Two or more of M, N, Y, K, and D which are included in the second primer are each a degenerate base which specifies two or more kinds of bases, and the second primer includes at least 10 kinds of polynucleotides including at least the first to 15th bases in the base sequences of SEQ ID NOs: 3 to 12.