A float type drain trap in which an inlet, a valve chamber, and an outlet are formed in a valve casing, a valve seat member, a discharge passage, a float, an operation member, a valve member that communicates between the periphery anterior to and the periphery posterior to the middle portion of the operation member when the operation member is in the advanced position and that blocks the periphery anterior to and the periphery posterior to the middle portion of the operation member from one another when the operation member is in the most retracted position is provided to the middle portion of the operation member, and a blow passage formed to communicate the periphery posterior to the valve member of the operation member with the outside distinct from the outlet.