Death rate of accidental drowning in the bathtub was the highest among casualties occurring at home, according to the annual report of the Japanese Ministry of Health, Labour and Welfare in 2007. To prevent accidental drowning during bathing at home, we obtained respiratory waveforms from bioelectric impedance (BEI) measurement using non-contact electrodes. The BEI measurement is an involuntary measurement method, from which respiratory waveform during bathing can be extracted. In the present study, to find the most appropriate electrode configuration as well as the optimal measuring frequency, we calculated the frequency dependence of impedance amplitude by numerical technique based on a three-dimensional finite difference method for a composite system consisting of a human body submerged in bath water. The results of model calculation agreed with the experimental results. Next, to obtain respiratory waveforms with large amplitudes, we investigated the optimal frequency experimentally. The frequency of 1MHz was suitable for involuntary measurement of respiratory waveform during bathing.