[Problem to be Solved]Provided are a stress buffer layer which satisfactorily releases stress acting on a joint portion, allowing an improvement in mounting reliability, and a method for producing the stress buffer layer.[Solution]A stress buffer sheet 10 is constituted by arranging external conductive layers 16A and 16B on the front and rear main surfaces of a through electrode layer 13 in which a resin 12 is filled between many columnar internal electrodes (metal pillars) 14. The columnar internal electrodes 14 are formed using a porous oxide base material 30 formed by anodic oxidation of valve metal; the oxide base material 30 is selectively removed after the internal electrodes 14 have been formed, and the resin 12 having a Young's modulus smaller than the oxide base material 30 is filled in a resultant void space. The internal electrode 14 has a high aspect ratio and a remarkable flexibility. The resin 12 has a small Young's modulus and can be deformed together with the internal electrode 14. Accordingly, in a structure having a wiring board 20 and an electronic component 24 connected through the stress buffer sheet 10, when stress acts on the joint portion during mounting of the electronic component 24, the whole of the through electrode layer 13 is deformed so that the stress is absorbed or released.