An oxygen concentration sensor of this invention comprises an element and a diffusion hole. The element is exposed to a gas to be measured and has a chamber portion formed together with an oxygen ion conductive solid electrolyte. The diffusion hole is provided for restricting the flow of said gas to be measured into said chamber portion. Furthermore, the relationship 1.ltoreq.S/(L.multidot.V).ltoreq.5 is established with the diffusion hole having a cross sectional area S (mm.sup.2) and a length L (mm) and the chamber portion having a volume V (mm.sup.3). To do so, the oxygen concentration sensor can provide a stable output even if a pressure of the gas to be measured is abruptly changed.