It is well known that oxygen transfer in an irrigation and drainage system occurs on the flow surface by reaeration as agricultural water flows down the waterway, and that water management structures, such as drops or weirs, largely improve this oxygen transfer. We focus on a labyrinth weir, where an improvement of the aeration function is expected because the crest length per unit channel width is longer than that of a full-width rectangular weir. The oxygen transfer at a labyrinth weir was clarified from reaeration experiments in a circular channel. The aeration function of a labyrinth weir improves as the crest becomes longer, and the maximum value of the function is reached at approximately 1.6 times the channel width. Furthermore, the predicted formula of reaeration with the upstream energy of the weir, the tail water depth, the function which expresses the influence of the crest length of the labyrinth weir, and the high degree of conformity were obtained. The labyrinth weirs do not require a large fall to produce the same aeration effect, as compared with conventional drops or weirs. The labyrinth weir is effective for the efficient uptake of oxygen into agricultural water.