Daily electrocardiogram (ECG) monitoring is helpful for the early detection of cardiopulmonary disorders. In particular, bathing poses a risk for abnormal cardiac beats, respiratory failure, and drowning owing to the thermal effect and water pressure on the body Thus, there is a need for nonintrusive measurement techniques without the attachment of electrodes and any instrument operation, that is, a bathtub ECG monitoring system. In this paper, we describe the development of a new bathtub ECG monitoring system that has electrodes pasted outside the bathtub wall, and thus the capacitive coupling was made from the electrodes to the tap water through the bathtub wall. In a previous system, the decrease in the thickness of the bathtub wall and the location of a long-tape-type reference electrode to prevent oscillation by the incorporation of environmental noise were required to obtain a stable ECG signal. To resolve these drawbacks, two electrodes covered by an active shield with an amplifier were pasted to the outside surface of the bathtub wall without the decrease in thickness, near the bather’s right scapula and left hip. To prevent the signal baseline fluctuation and oscillation caused by the environmental noise, the most suitable input impedance of the amplifier was also determined. In 13 healthy subjects (21.9 ± 1.94 years), the QRS components in ECG were successfully detected during bathing with a reasonable signal-to-noise ratio of more than 14.9 dB. Moreover, the intervals of the heartbeat and respiration obtained by the bathtub system and by the direct method agreed with each other.