Radon daughter concentrations in outdoor and indoor air were measured at the campus of Fukushima Medical College. Radon daughters in the air were sampled with a flow rate of 20 L/min through a glass fiber filter or a membrane filter, and the decay of activity on the filter was analyzed with two types of technique, i.e., the gross-beta one-count method using a G.M. tube and the alpha spectroscopic method using a semiconductor detector. The present paper describes principles and techniques of the measurement in detail.