Optical-fiber preform and method for manufacturing optical-fiber preform

Provided is an optical-fiber preform that can be drawn into a low-loss optical fiber. This optical-fiber preform (1) comprises a core section (10) and a cladding section (20) that surrounds the core section (10). The core section (10) contains a first core section (11) and a second core section (12) that surrounds the first core section (11). The cladding section (20) contains a first cladding section (21) that surrounds the second core section (12) and a second cladding section (22) that surrounds the first cladding section (21). The first core section (11) contains an alkali metal. In some or all regions in which the concentration of alkali-metal atoms is greater than or equal to 100 atomic ppm, the concentration of oxygen molecules in the glass is between 30 and 200 mol ppb, inclusive, and in regions in which the concentration of alkali-metal atoms is less than or equal to 50 atomic ppm, the concentration of oxygen molecules in the glass is less than or equal to 10 mol ppb.