The article deals with the issue of an important element of proving the invalidity of marriage on the grounds of total simulation of marital consent (Canon 1101 § 1), which is the cause of simulation. The subject of the analysis are all the decisions pro nullitate matrimonii of the Metropolitan Tribunal of Cracow which were given in the years 2010-2020 on the above ground. In the first part, the importance of the causa simulationis at the instruction stage in the canonical process is presented. The causa celebrandi and causa simulandi occurring in the presented jurisprudence form the essential part of this study. The author also proposes some legal and pastoral measures that may serve to eliminate instances of contracting invalid marriages. In conclusion, proving the causa simulandi is an important aid for judges to achieve moral certainty about the nullity of a marriage on the grounds in question.