It is known that there is a front-like structure at the mixed layer depth (MLD) distri- bution in the subtropical gyre, which is called the MLD front, and is associated with the formation region of mode water. In the present article, the generation mechanism of the MLD front is studied using an idealized ocean general circulation model with no seasonal forcing. First, it is shown that the MLD front occurs along a curve where u g ·∇ ∇ ∇ ∇ ∇ T s = 0 is satisfied (u g is the upper ocean geostrophic velocity vector, T s is the sea surface temperature and ∇ ∇ ∇ ∇ ∇ is the horizontal gradient operator). In other words, the front is the boundary between the subduction region (u g ·∇ T s > 0) and the region where subduction does not occur (u g ·∇ T s < 0). Second, we have investigated subduction of low potential vorticity water at the MLD front, which has been pointed out by past studies. Since u g ·∇ ∇ ∇ ∇ T s = 0 at the MLD front, the water particles do not cross the out- crop at the MLD front. The water that is subducted at the MLD front has come from the deep mixed layer region where the sea surface temperature is higher than that at the MLD front. The temperature of the water in the deep mixed layer region de- creases as it is advected eastward, attains its minimum at the MLD front where u g ·∇ ∇ ∇ ∇ ∇ T s = 0, and then subducts under the warmer surface layer. Since the deep mixed layer water subducts beneath a thin stratified surface layer, maintaining its thick- ness, the mixed layer depth changes abruptly at the subduction location.