An attempt has been made to establish an approximate method to calculate the resistance in crease of a full ship in regular oblique waves. Applicability of this method was confirmed by the model test carried out in regular oblique waves making use of a newly designed dynamometer. Comparing the measured resistance increase and the computed one in accordance with the slender body assumption, fairly good agreement is shown as a whole except for the range of shorter wave length where the measured value is larger than the computed one. An approximate calculation method was proposed in consideration of the resistance increase due to the wave reflection at the blunt bow. It was shown that total resistance increase of a full ship in waves can be obtained approximately by the sum of the resistance due to ship motions and the wave reflection at the bow.