The present invention relates to an agent for preventing and treating benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS) associated with BPH and a relaxant for relaxing urethral smooth muscle or prostatic smooth muscle comprising a pyrazolopyrimidinone compound as an effective ingredient. The agent in accordance with the present invention can provide nitric oxides by inhibiting the activity of PDE-5 that decomposes c-GMP, and the provided nitric oxides relax the urethral smooth muscle or the prostatic smooth muscle to lower the intraurethral pressure (IUP), thus treating BPH and LUTS associated with BPH. Furthermore, the time required for reaching a maximum blood concentration is shorter and the half-life is longer than the other PDE-5 inhibitors, thus reducing the frequency of administration. Moreover, the agent of the invention causing few side effects can be efficiently used as a safe drug.