We investigated the microcontainer shape in electropolymerization of polypyrrole (PPy) on bubbles using real-time microradiography. We revealed the existence of a "deformation force" at the three-phase boundary among gas (bubble), liquid (electrolyte), and solid (PPy). As the deformation force increases, the microcontainer shape gradually changes from spherical to elliptical and then to cylindrical. The force is proportional to the polymerization rate that is enhanced by the applied voltage and monomer concentration.