To provide low scattering silica glass suitable as a material of an optical communication fiber.Silica glass, which has a fictive temperature of at least 1,000° C., and which has a void radius of at most 0.240 nm as measured by positron annihilation lifetime spectroscopy. A method for heat-treating silica glass, which comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 30 MPa, and cooling the silica glass at an average temperature-decreasing rate of at least 40° C./min during cooling within a temperature range of from 1,200° C. to 900° C. A method for heat-treating silica glass, which comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 140 MPa, and cooling the silica glass in an atmosphere under a pressure of at least 140 MPa during cooling within a temperature range of from 1,200° C. to 900° C.