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 %,;KaZrb(PO4)3 (1);wherein, in Formula 1, a is a positive number of from 0.8 to 1.2 and b is a positive number satisfying a+4b=9.
