To provide heat-expandable microspheres characterized in that even after they are mixed with base materials, the content of the microspheres can be measured.

What are provided are heat-expandable microspheres which are ones having a structure formed by entrapping a blowing agent in a shell made from a polymer and is characterized in that when measured on a water extract obtained by steps 1 and 2: step 1 of dispersing 5g of the heat-expandable microspheres at 25C in 20 g of ion-exchanged water of a pH of 7 and an electric conductivity of 1 to prepare a dispersion and step 2 of shaking the dispersion for 30 min at the same temperature to perform water extraction, the difference between 2 and 1 (wherein 2 is the electric conductivity measured at 25C) is greater than 1 mS/cm, and a composition of the microspheres. Further is provided is a measurement method comprising estimating the content of the heat-expandable microspheres in the composition according to the electric conductivity of the composition.