Human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) are significantly immature compared with adult cardiomyocytes and exhibit a fetal-like phenotype. Although efficient propagation of electrical signals in the heart is crucial for natural development program and functionality, maturation of hiPS-CMs causes hyperpolarization of resting membrane potentials and a loss of automaticity in vitro. We therefore investigated whether artificial electrical field stimulations mature physiological properties of hiPS-CMs, especially on intracellular sodium concentrations ([Na⁺]_i). We cultured hiPS-CMs for a week under electrical field stimulations (3 V/cm, 1 ms, 1 Hz), and quantified [Na⁺]_i with a Na⁺ indicator. Basal　intracellular sodium concentration without stimuli was 4.7 mM (n = 17) which is much lower than that in normal excitable cells (10-15 mM). The intracellular sodium concentration with the electrical stimuli (5.7 mM, n = 23) was significantly greater than [Na⁺]_i without stimuli. These results suggest that artificial electrical stimulations enable us to obtain more mature hiPS-CMs in vitro.