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We are developing S-CAT computer test system that will be the first automated adaptive speaking test for Japanese. The speaking ability of examinees is scored using speech processing techniques without human raters. By using computers for the scoring, it is possible to largely reduce the scoring cost and provide a convenient means for language learners to evaluate their learning status. While the S-CAT test has several categories of question items, open answer question is technically the most challenging one since examinees freely talk about a given topic or argue something for a given material. For this problem, we proposed to use support vector regression (SVR) with various features. Some of the features rely on speech recognition hypothesis and others do not. SVR is more robust than multiple regression and the best result was obtained when 390 dimensional features that combine everything were used. The correlation coefficients between human rated and SVR estimated scores were 0.878, 0.847, 0.853, and 0.872 for fluency, accuracy, content, and richness measures, respectively.