It is difficult to use scanning electron microscopy to observe the structure and movement of biological tissue immersed in solution. To enable such observations, we created a highly deformable and electron-transmissive polyimide film that can withstand the pressure difference between the high-vacuum electron column and atmospheric-pressure sample chamber. With this film, we used scanning electron microscopy to measure the intrinsic fine structure and movement of the contractile fibers of excised mouse heart immersed in physiological solution. Our measurements revealed that the excised heart is a dynamic tissue that undergoes relaxation oscillation based on a three-dimensional force balance.