Purpose: To find out deaf pupil's ability to use auxiliary words and verbs which is an important element in their development of the Japanese language, and to seek suggestions to the planning of instruction for them. Procedure: Performance test of sentences which are to be filled up with auxiliary words and verbs was given to 179 secondary deaf school pupils (53 lower secondary, 126 upper secondary pupils) and 197 secondary hearing school pupils (98 lower secondary, 99 upper secondary pupils). The ratios of the correct use of each auxiliary word, and verb were brought out and distributed into five step intervals. Abstruct of the result: 1. In deaf groups, most of ratios of the correct using of each auxiliary word failed into 0-20.0 step interval, and few of them entered in 60.1-80.0 or 80.1-100 step intervals. On the other hand, in hearing groups most of the ratios entered in 80.1-100 step interval and few of them failed in 0-20.0 step interval. 2. Most of ratios of the correct using of each auxiliary verb were scattered in 0.-20.0 step interval or 20.1-40.0 step interval in deaf groups and the ratios which entered in 60.1-80.0 or 80.1-100 step interval were far more in hearing groups than in deaf groups. 3. Therefore, such ability of lower and upper secondary deaf school pupils tend to be inferior to that of hearing pupils of the first grade of lower secondary school. 4. Errors of deaf pupils at the test of auxiliary words are caused by (a) imperfect discrimination of surd sound and sonant. (b) omission of letter. (c) hypermetre (d) impemfect considemation to the connection of words. (e) using of other parts of speech. (f) immatureness of ideas. (g) to seize partialiy the line of thought. (h) subjective embeliighment to the line of thought. 5. Ermors at the test of auxiliary verbs are caused by (the same of aboves are excluded) (a) mistake of inflextion. (b) misupderstanding of tenses. (c) displacement of conjecture to designation. (d) confusion of passive and afformation. Suggestions for the planning of instruction for the deaf children. 1. To prevent deaf children from acquiring sign language. 2. Practice of modes of expression as well as extending vocabularies. 3. Training of readmg and descriptive abilities. 4. Training of ductive and inductive thinking.