In an image forming apparatus having a powder transport mechanism including a powder container and a powder transport means which rotates relative to the powder container, the powder transport means has the rotation center substantially on the axis of the powder container, and the space between the inner wall surface of the powder container and the powder transport means is sealed with magnetic particles held by a magnetic-filed generation means. The magnetic particles have a specific saturation magnetization s and a specific value of A_A where the residual magnetic flux density of the magnetic-field generation means is represented by A in a specific range and the magnetization intencity of the magnetic particles in a magnetic field with the residual magnetic flux density A is represented by _A.