A thermally foamable microsphere having a structure that a foaming agent is encapsulated in an outer shell formed from a polymer, wherein supposing an electric conductivity of a water extract obtained by Step 1 of dispersing 5 g of the thermally foamable microsphere in 20 g of ion-exchanged water having a pH of 7 and an electric conductivity of .sigma.1 at a temperature of 25.degree. C. to prepare a liquid dispersion; and Step 2 of shaking the liquid dispersion at the same temperature for 30 minutes to conduct a water extraction treatment, as measured at 25.degree. C. is .sigma.2, a difference .sigma.2-.sigma.1 between .sigma.2 and .sigma.1 is at most 1 mS/cm, and a production process including measuring an electric conductivity of a filtrate in the washing step to obtain a thermally foamable microsphere exhibiting a desired electric conductivity on the basis of a previously prepared relational expression between the electric conductivity of the filtrate and the electric conductivity of a water extract of the thermally foamable microsphere.