Provided are: a muffler that can suitably obtain an attenuation effect of a sound wave; and a muffling device including the same. The muffler is installed on a wall surface of a pipe conduit where a gas flows. The muffler includes: a front surface exposed to an inner surface of the pipe conduit; a bottom surface facing the front surface; and a side surface disposed between the front surface and the bottom surface and covering a space between the front surface and the bottom surface. The front surface is formed with a plurality of through-holes, and the number of through-holes arrayed in a gas flow direction is one or two, and the number thereof arrayed in a direction orthogonal to the gas flow direction is 10 or more.