To solve the problem that temperature rise of oil for dehydration treatment in a cooker of large capacity takes time and when the inside of the cooker is decompressed, bumping or the like occurs and a foreign matter is pulled in from a steam suction opening to reduce suction efficiency or to cause contamination of impurities in effluent.

A ceiling wall among outer peripheral side walls of a cooker 1 main body is formed projectingly above a rotating track of a stirring blade having an axis suspended inside to form a steam storing space 17 between the rotating track of the stirring blade and the ceiling wall. At the projectingly formed ceiling wall, a steam suction opening 19 is formed to suck steam gathered in the steam storing space 17 and a jacket part 40 of the cooker 1 main body is formed in the outer peripheral side walls of the cooker main body except the projectingly formed ceiling wall facing the steam storing space 17. Further, an oil splash catching septum 31 for catching oil splashes are provided in the steam storing space 17.