A formation method for forming a water repellent layer on a surface of a metal substrate forms asperities on a surface of a metal basis material of the metal substrate by irradiating the metal basis material with plasma ions. The formation method forms an alloy from atoms of a metal of the metal basis material and the plasma ions. The formation method forms the asperities with portions of the metal basis material, which are not etched due to the alloy, and portions of the metal basis material, which are not alloyed but are etched. The formation method forms the water repellent layer by forming the asperities.