This study examines the application of the ius curia novit principle in the legal interpretation of pretrial single judge decisions that examine the validity of a person's determination as a suspect which is very obvious from the decision of the first and final District Court as the first and last dispute breaker. This research belongs to the type of doctrinal research with a statutory approach. This study concludes, among the decisions of the District Courts that adjudicate pretrial cases there are decisions that only examine formal matters or that prioritize objective elements, namely limited to administrative problems that have been carried out by investigators, on the other hand there are decisions that are more daring. Into the subjective element, which includes considering the quality of the examination that has been carried out. This means that there is still a disparity in the judge's decision in testing the validity of the determination of the suspect. Therefore, this study will examine the application of the ius curia novit principle in the decisions of the pretrial single judge who examines the validity of the determination of a person as a suspect. The result of the research is that the application of the ius curia novit principle in the examination at the Constitutional Court confirms that the judge cannot refuse to examine a case because the law does not exist or is not clear. Judges are considered to know the law in understanding the legal settlement of cases submitted to them, based on the principles of propriety, legal certainty, and justice.