A storage device includes a first conductive layer and a second conductive layer, with an intermediate layer therebetween. The intermediate layer includes a first and second compound regions. The first compound region includes first and second adjacent portions and the second compound region includes third and fourth adjacent portions. Electrical resistance between the first and second conductive layers changes according to a polarity applied across the intermediate layer. In a first polarity state, a concentration of a first element in the first portion is higher than a concentration of the first element in the second portion of the first compound region. A thickness of the third portion in the first polarity state is greater than the thickness of the fourth portion in the first polarity state.