Abstract: The present invention relates to an apparatus for preventing mud from being hardened in a drain holding tank, in an apparatus that treats drains generated during the marine drilling of petroleum or gas. The apparatus for preventing mud from being hardened in a drain holding tank includes a drain holding tank which gathers a drain being generated in a drilling process; a mud re-compounding pump which circulates a fluid having a pressure that is higher than a predetermined value by spouting the fluid to an area of the drain holding tank where the mud is stored, to prevent mud included in the drain from being hardened in the drain holding tank; a transfer pump which transfers the mud; and a waste mud tank, configured to gather the mud. The apparatus for preventing mud from being hardened in a drain holding tank can circulate the mud at an area of the drain holding tank in which the mud is stored for a period of time, to prevent the mud from being hardened.