We investigated the validity of estimation of usual mean salt intake using overnight urine using an electronic salt sensor. Measurement was performed in four healthy young women for a period of 31 days with measurement of 24-hour urinary salt excretion as a standard. We also investigated the correlation between measurement error and the time of the latest meal before measurement, and 24-hour urinary sodium: potassium ratio (Na/K). With the exception of one subject, a significant correlation was observed between mean 24-hour urinary salt excretion and estimated values by the electronic salt sensor. The electronic salt sensor underestimated daily salt excretion in subjects who had a high frequency of days with less than approximately 6g/day of urinary salt excretion. The estimation error was influenced by the length of the interval between the subject’s latest meal and the starting of overnight urine collection, and by Na/K in 24-hour urine. It is important to consider the possibility of those errors when using an electronic salt sensor in nutritional assessment or when monitoring day-to-day variation in individualized guidance for salt reduction.