As one of the basic areas of research involving the relationship between health and electromagnetic environments, the authors systematically measured the horizontal power density P_H of electromagnetic waves (acting as plane waves) employed in typical digital cordless phones versus the horizontal distance l_H from the source of the digital cordless phone in the call state. During these measurements noticeable errors were found in the values of P_H for short distances (l_H＜0.5 m). The adaptive limit of the inverse square law for electromagnetism for short distances has previously been unknown. To clarify this, the present research examines a method for estimating the value of P_H for short distances.