In a suburb of Tokyo, the concentrations of222Rn, its progeny and aerosol particles in outdoor air were measured simultaneously from December 1993 to April 1995. Values of 0.69 and 0.024 were obtained for annual mean values of equilibrium factor, F, and unattached fraction of potential alpha energy, fp, respectively. The concentrations of222Rn were found to be high in winter and low in summer and to be high from night to morning and low during daytime. On the other hand, no clear seasonal and diurnal variations were observed in F and fp. The variations of fp and unattached fraction of218Po, fA, strongly depended on aerosol concentration. In cases where the aerosol concentrations were lower than 10000 cm-3, it was found that values of F were relatively low and values of fp and fA were fairly high. However, such cases were only about 3% of all data observed in 1994. From the relation between fA and aerosol concentration, the representative value of aerosol particle diameter was estimated to be 0.1-0.15 μm.