A filler that can suppress thermal expansion of a glass composition with a small amount thereof added and is also excellent in terms of flowability when the glass composition is melted, and a glass composition containing the filler are provided. There is also provided a process for producing a hexagonal phosphate-based compound that can be suitably used as the filler using a simple, industrially advantageous method.;The filler of the present invention contains a hexagonal phosphate-based compound that has a purity of 90% or higher and is represented by the following Formula 1, the filler having a content of an ionic compound that is no greater than 1.0 wt %,
