For the purpose of selecting items in a test, this study presented a viewpoint of psychologically optimal difficulty level, as well as measurement efficiency, of items. A paper-and-pencil test (P & P) composed of hard, moderate and easy subtests was administered to 298 students at a university. A computerized adaptive test (CAT) was also administered to 79 students. The items of both tests were selected from Shiba's Word Meaning Comprehension Test, for which the estimates of parameters of two-parameter item response model were available. The results of P & P research showed that the psychologically optimal success level would be such that the proportion of right answers is somewhere between .75 and .85. A similar result was obtained from CAT research, where the proportion of about .8 might be desirable. Traditionally a success rate of .5 has been recommended in adaptive testing. In this study, however, it was suggested that the items of such level would be too hard psychologically for many examinees.