To provide a method for producing preliminarily foamed particles of a polyolefin-based resin, which are stably dispersed in a dispersion medium and characterized in that a dispersing agent sticking on the surface of each particle does not inhibit mutual fusion during molding and does not contaminate or corrode a mold.

The method for producing the preliminarily foamed particles of a polyolefin-based resin, comprises charging a dispersion containing polyolefin-based resin particles, water, a dispersing agent and a dispersing aid, and a foaming agent into a pressure-resistant container, then heating the inside of the pressure-resistant container to a prescribed temperature under pressure, and releasing the dispersion into an atmosphere having a pressure lower than that in the pressure-resistant container while keeping the temperature and pressure constant, wherein the dispersing agent is magnesium phosphate and the pH of the dispersion in the pressure-resistant container is 9.