A water depth measuring method comprises steps of making a bundle of reference water column pressure measuring tubes of a U character form of the number of n≧2 at a same length with a water depth measuring tube in order to compensate for a total error of a water depth measurement; filling an amount of water into the reference water column pressure measuring tubes to form water columns therein; measuring the water pressure in the reference water column pressure measuring tube by a pressure transducer before measuring a water depth; subtracting previously known water column pressure values derived from the measured water pressure values to calculate total error values; calculating a total error based on the total error values, in which the total error was generated upon measuring of the bubble generating pressure in the water column pressure measuring tubes; subtracting the total error from the bubble generating pressure, dividing the calculated water column pressures by the water column pressure in the reference water column pressure measuring tube having the most approximate value thereto and multiplying the re-calculated value by the water column pressure thereby to measure a water depth in a higher accuracy.