Although many experiments in irregular waves are carried out at towing tanks, these experiments have been generally carried out without the explanation how long the measuring of waves, ship motion and/or added resistance etc. are required. As the necessary record length effects on the size of towing tanks and model ships and also the power estimation of wave maker, it should be enough investigated. Since under the present situation as experimental accuracy has been discussed severely, this investigation is essential. The matters of experimental accuracy include the accuracy of facilities, measuring instruments and also the accuracy of the data obtained in limited time. The former problem has been studied in many ways but the later problem is still insufficient. We research this problem under some assumptions and apply that procedure to estimate an accuracy of the sea wave-correction for ship trial experimental results, which is performed in only one mile interval.