To provide a porous vitrified bond grindstone, improved in anti-slipout properties of abrasive grains to achieve long service life thereof without damaging the truing performance and dressing performance.

The porous vitrified bond grindstone is formed of: the abrasive grains 1; a vitrified bond bridge 2 formed by tying the abrasive grains 1 together by a vitrified bond; and pores. The pores includes external pores 3 present outside the vitrified bond bridge 2, and closed pores 4 encapsulated in the vitrified bond bridge 2. Then the total volume of the external pores 3 outside the vitrified bond bridge 2 and the closed pores 4 encapsulated in the vitrified bond bridge 2 is set at 5 to 40 (preferably 23 to 43) vol.% with respect to the total volume of the grindstone. The volume of the closed pores 4 encapsulated in the vitrified bond bridge 2 is set at 10 to 50 (preferably 7 to 44) vol.% with respect to the volume of the vitrified bond.