An etching method comprising: placing a base layer and a polymer layer formed on the base layer on a mounting table configured as a lower electrode within a processing chamber, the polymer layer having a periodic pattern of a first polymer and a second polymer formed by self-assembling the first polymer and the second polymer of a block copolymer that is capable of being self-assembled; etching the polymer layer such that the second polymer is removed and a pattern of the first polymer is formed as a mask, wherein the first polymer is polystyrene and the second polymer is methyl methacrylate, wherein the etching the polymer layer includes: introducing an Ar/O 2 gas into said processing chamber; setting a frequency of a high frequency power supply to be higher than or equal to 60 MHz, such that a great amount of the oxygen ion energy is distributed within a range from 15 eV to 30 eV, which is smaller than ion energy for generating an etching yield of the first polymer and equal to or larger than ion energy for generating an etching yield of the second polymer, and supplying the high frequency power to the mounting table from the high frequency power supply; generating plasma from the gas introduced in the processing chamber by applying the high frequency power; and etching the polymer layer by using the generated plasma. placing a base layer and a polymer layer formed on the base layer on a mounting table configured as a lower electrode within a processing chamber, the polymer layer having a periodic pattern of a first polymer and a second polymer formed by self-assembling the first polymer and the second polymer of a block copolymer that is capable of being self-assembled; etching the polymer layer such that the second polymer is removed and a pattern of the first polymer is formed as a mask, wherein the first polymer is polystyrene and the second polymer is methyl methacrylate, wherein the etching the polymer layer includes: introducing an Ar/O 2 gas into said processing chamber; setting a frequency of a high frequency power supply to be higher than or equal to 60 MHz, such that a great amount of the oxygen ion energy is distributed within a range from 15 eV to 30 eV, which is smaller than ion energy for generating an etching yield of the first polymer and equal to or larger than ion energy for generating an etching yield of the second polymer, and supplying the high frequency power to the mounting table from the high frequency power supply; generating plasma from the gas introduced in the processing chamber by applying the high frequency power; and etching the polymer layer by using the generated plasma. introducing an Ar/O 2 gas into said processing chamber; setting a frequency of a high frequency power supply to be higher than or equal to 60 MHz, such that a great amount of the oxygen ion energy is distributed within a range from 15 eV to 30 eV, which is smaller than ion energy for generating an etching yield of the first polymer and equal to or larger than ion energy for generating an etching yield of the second polymer, and supplying the high frequency power to the mounting table from the high frequency power supply; generating plasma from the gas introduced in the processing chamber by applying the high frequency power; and etching the polymer layer by using the generated plasma.