The prognosis of primary cutaneous diffuse B-cell lymphoma, leg type (PCDLBCL-LT) is poor, especially in patients with multiple lesions, which usually requires systemic chemotherapy as treatment. Here, we report a case of PCDLBCL-LT with multiple lesions and spontaneous regression after biopsy. A 66-year-old man was referred to our clinic with a half-year history of multiple plaques on his back and flank. He had hypertension and had been treated with olmesartan, but he did not have a history of autoimmune diseases, such as rheumatoid arthritis, and no medical history of using immunosuppressive agents. On physical examination, multiple annular to arcuate plaques, up to 4-cm diameter, were found in his back and left flank. Superficial lymph nodes were not palpable. A skin biopsy specimen from the lower and upper back showed bottom heavy dense nodular infiltrates with large atypical cells and many small inflammatory cells. Atypical cells were positive for CD20, CD79a, and bcl-2; partially positive for bcl-6; but did not show staining for MUM-1 or CD10. Small lymphoid cells were positive for CD3, and cytotoxic molecules, such as TIA-1 and perforin, were also positive. Monoclonal immunoglobulin heavy chain rearrangement was detected using Southern blotting. Epstein-Barr virus-encoded sRNA was not detected in the tumour using in situ hybridization. Position emission tomography and enhanced computed tomography did not detect any nodal and visceral involvement. We diagnosed the patient as having PCDLBCL-LT (T2, N0, M0). Most of the lesions, including un-biopsied ones, disappeared after biopsy; however, the plaque on the left flank remained. We performed skin biopsy on the remaining lesion, and its pathology was almost similar to those of the previously biopsied lesions. However, this lesion still remained after biopsy; therefore, it was treated with radiation therapy. The patient remained free from recurrence or distant metastasis for a year.