PURPOSE: To obtain a constant quality over a long range by controlling the concn. of the ammonium ion generated in a nitric acid electrolyte to a predetermined range.

CONSTITUTION: The concn. of the ammonium ion generated in a nitric acid electrolyte is controlled to a range of 0-300mg/l. That is, an electrolyte based on nitric acid is used and the concn. of the ammonium ion in said electrolyte is controlled to a certain concn. or less without adding an additive. Since the shape of a pit obtained by electrolytic surface roughening is changed by an increase of the ammonium ion in the electrolyte, the surface roughening shape of an aluminum support subjected to electrolytic surface roughening in the longitudinal direction thereof becomes non-uniform and the capacity as the support for a printing plate varies in the longitudinal direction. Therefore, by controlling the concn. of the ammonium ion in this electrolyte within a definite range in matching relation to other electrolytic surface roughening condition, stable electrolytic surface roughening can be performed over the longitudinal direction of an aluminum support for a printing plate.