The purpose of this study was to investigate the reality of principals\' job performance in public middle school. For that purpose, 5 principals were interviewed deeply, who are working in Korean public middle school. The results of this research were as followings. Principals have experienced \"enjoying principalship\", \"working as administrative manager\", \"doing the restrictive autonomy\", \"taking a compromising attitude to teachers\", \"depending on personal experiences\". And I have found the school culture embeded in principals\' job performance : \'the culture of administrative organization\', \'the culture of community\', \'the culture of anti-supervision\', \'the culture of paternalism\'. From this research, I have some hints for principal policy. 1) We need to retrieve the principals\' supervisionary function. 2) We have to develop the principals\' political leadership. 3) We need to strengthen authority and responsibility of principals.