A method for producing a high purity silica sol is provided. This method has enabled use of water glass for the starting material, and the resulting silica sol has a reduced metal impurity Cu and Ni content compared to conventional methods. The method comprises (1) ultrafiltration of an aqueous solution of an alkali silicate; (2) ion exchange process for removal of at least a part of cationic components in the purified aqueous solution of an alkali silicate; (3) another ion exchange process using a chelating ion exchange resin to obtain high purity silicate solution; and (4) adjustment of a part of the high purity silicate solution (seed solution) to alkaline pH and mixing of this solution with another part of the solution (feed solution) to produce a high purity silica sol having a Cu concentration and a Ni concentration (in relation to the dry silica) of up to 50 ppb.