An accuracy of visible observation for waves at sea has been called in question because the visible observation is always and much affected by each observer's subjectivity. Accordingly, author analyzed the character of that observation by mean of simultaneous observation with many observers and wave gauge. As a result, next some characters were made clear. (1) Each observer's value of wave factors are inevitably different from each other. The distribution of that difference is approximately expressed by Weibull distribution. (2) The width of that difference distribution depends on wave profile. (3) An accuracy of observation for wave which steepness is less than 1% is worse because the width of difference distribution is comparatively more broad. (4) An accuracy of wave period observation is not good. Because observed wave period is evidently affected by ship encouter relative wave period. (5) An accuracy of wave length observation is almost same with one of wave height observation. In these results, we should use the data of visible observation with knowledge of above characters. And the wave length observation should be hereafter adopted instead of wave period observation.