A solid phosphoric acid catalyst which, when gas-phase synthesizing trioxane from formaldehyde, results in production of trioxane at a high yield and at a high selection rate by markedly suppressing side reactions, reduces the adhesion of organic matter to the catalyst surface, and achieves a stable and long-term catalyst reaction. This solid phosphoric acid catalyst contains silicon phosphate oxide (SPO) obtained by sintering at a temperature of at least 200° C. a carrier material to which a phosphoric acid component has been added, and the total quantity of acid does not exceed 5 mmol per 1 g of solid phosphoric acid catalyst when calculated as acid quantity determined by NH3-TPD measurement.