To provide a practical vessel type iron electrolyzer for an iron ion feeder, which is simply mounted and is free from insulation defect, contact failure or the like.

In the vessel type iron electrolyzer 1 for the iron ion feeder in which one unit of a cartridge electrode is formed by fixing (n) pieces of iron electrode plates 5, 6 arranged nearly in parallel with a certain interval and an energizing means 9 with fastening parts and one or more cartridge electrodes are fixed to an electrolytic cell 2 to elute iron ion in an electrolyte aqueous solution, (n) is an integer of ≥3 and the absolute value of the difference between the width of a (n-1)th iron electrode plate and the width of a nth iron electrode plate is 10-30% of the width of a wide electrode 5.