The present invention is to provide an electrolyte membrane which retains durability even after having undergone a dimensional change accompanying chemical deterioration caused owing to radicals having high oxidizing ability such as hydroxyl radicals (.OH) or peroxide radicals (.OOH), and the selecting method thereof.An electrolyte membrane has an amount of dimensional change in a plane direction between dimensions obtained in a dry state before and after carrying out the Fenton test, the amount of dimensional change before and after the Fenton test being smaller than an amount of maximum elastic deformation in a plane direction obtained in dry state before carrying out the Fenton test, provided that the Fenton test is carried out under the following condition, and the selecting method of the same. (1) iron ion (Fe2+) concentration: 4 ppm, (2) hydrogen peroxide concentration: 3 wt %, (3) boiling temperature: 80° C., (4) boiling time: 120 minutes.