反トラスト法
Joustra was born in Hengelo and studied law at the University of Groningen. He graduated in 1975, on a thesis on United States antitrust law. He then joined the Ministry of Agriculture, Nature and Food Quality legal department. In 1987 he was made secretary-general of the ministry, becoming the youngest secretary general of a Dutch ministry ever (aged 35). He remained in this post until December 2001, serving under five different ministers.