The exposure is carried out at100 to 2,000 mJ/cm.sup.2, preferably 250 to 1,000 mJ/cm.sup.2.The development can be carried out by a known method such as a spraying method or a dipping method, and as a developer solution, there can be used a known dilute alkali aqueous solution, such as a sodium carbonate aqueous solution, a sodiumhydr