In general, the dopant concentration distribution formed by ion implantation has a normal distribution (or, a Gaussian distribution) that spreads along the ion traveling direction in an ion implantation target. In the example shown in FIG. 5, the dopant concentration distribution has a certain local maximum value in the variable resistance material 32. In FIG. 5, the dopant concentration distribution has a peak in the lower electrode 31. This is because the variable resistance material 32 and the lower electrode 31 are made of different materials. In the description below, for convenience of description, the maximum concentration in the variable resistance material 32 included in the set of variable resistance material 32 and lower electrode 31 will be referred to as a peak concentration. In general, the dopant concentration in an ion implantation target is almost zero in the vicinity of the surface from which the ions enter. The closer to the inside of the ion implantation target from the ion entering surface, the higher will be the dopant concentration. Likewise, the dopant concentration is zero at a position that is away from the ion entering surface of the set of variable resistance material 32 and lower electrode 31 in the depth direction. In the example shown in FIG. 5, the dopant concentration is zero in the bottom surface of the lower electrode 31.