In Korea, most apartment bathrooms use an under-slab-piping method, causing drainage to occur through the ceiling of the lower floor when the upper floor bathroom is used, inevitably leading to drainage noise issues. Especially in older apartment buildings, noise reduction measures have not been considered, resulting in significant harm to residents. This study aims to apply noise reduction technology to older apartment buildings vulnerable to bathroom drainage noise and verify its effectiveness. The target site is an apartment built in 1997, with a exclusive area of 84 m². The existing PVC drainage pipes, asbestos slate ceiling material, and wooden doors were replaced with low-noise PVC pipes, sound-absorbing ABS ceiling material, and ABS doors. The noise reduction performance was evaluated at maximum noise levels during toilet, sink, and bathtub drainage. Measurement results in the main bathroom showed a reduction of 22.9 dB(A) for toilet drainage noise and 16.6 dB(A) for sink drainage noise. In the family bathroom, reductions were 10.1 dB(A) for toilet drainage noise, 5.9 dB(A) for sink drainage noise, and 12.0 dB(A) for bathtub drainage noise. Applying this technology is expected to reduce living noise issues between neighbors in apartment buildings.