Surface-treated steel sheet

A surface-treated steel sheet, comprising:

• a steel sheet, and
• an alloy layer containing Ni and Co on a surface of the steel sheet,
• wherein
• in a case where, in a cross section obtained by cutting the alloy layer in a thickness direction, a range having a width of 2000 nm and extending from a surface of the alloy layer to a depth of 100 nm is partitioned into regions each having a width of 100 nm and a depth of 100 nm, within the range the alloy layer includes:
• a plurality of high Co concentration regions in each of which a ratio of a Co concentration to a sum of the Co concentration and a Ni concentration within the partitioned region is 0.8 or more, and
• a plurality of alloyed regions in each of which a ratio of the Co concentration to a sum of the Co concentration and the Ni concentration within the partitioned region is less than 0.8, the plurality of alloyed regions being arranged alternately with the high Co concentration regions.