To provide a method for foaming beads which have a light weight and a high compression strength even in a large average foam diameter, and to provide a method for manufacturing a foam thereof.

The thermoplastic resin bead foam is obtained by making an inert gas infiltrate in an unfoamed thermoplastic resin particle in a pressure vessel, thereafter releasing the pressure in the pressure vessel and taking out the unfoamed resin particle containing the gas having infiltrated therein, and heating and foaming the particle at temperature equal to or higher than the glass transition temperature of the resin constituting the particle, and has a numerical value σ₁₀/ρ of ≥5 obtained by dividing a 10% compression stress σ₁₀ (kPa) by a density ρ (kg/m³).