The present invention provides an agent to decompose unstable terminal group, which is effective in a small adding amount, which sufficiently decreases the quantity of residual unstable terminal group, and which generates very little limitation on the treatment method, the apparatus, and the use amount of the agent. Specifically, it provides a method for manufacturing stabilized polyacetal resin having the step of applying heat treatment to a polyacetal resin having an unstable terminal group in the presence of an agent to decompose the unstable terminal group, which agent is composed of a heterocyclic quaternary ammonium salt, thus decreasing the quantity of the unstable terminal group.