Kadima Treasurer Itzik Hadad and two other men were arrested on Tuesday, after the party filed a complaint of suspicious conduct to the police.
The three are suspected of theft by a manager, fraud, breach of trust, forgery of official documents and offenses.
Party leader Tzipi Livni asked the party's internal comptroller to investigate Hadad, who has been Kadima's treasurer since its establishment in 2006, due to discrepancies in the party's finances and a suspicion of conflict of interest.
Kadima legal adviser Eitan Haberman complained to the police and the State Comptroller's office earlier this month, after seeing the probe's results. The police then began a confidential investigation, which was announced to the public on Tuesday.