A developing device comprising: a developing container accommodating a developer containing toner and carrier particles; a cylindrical developing sleeve (103; 103A) rotatable while carrying the developer in said developing container; a magnet provided in said developing sleeve (103; 103A) and configured to generate a magnetic force for holding the developer; and a plurality of grooves (200; 300) provided at a developer carrying surface of said developing sleeve (103; 103A) and formed along a direction crossing a circumferential direction of said developing sleeve (103; 103A), wherein each of said grooves (200; 300) is formed by a bottom portion (201; 301) contacting the carrier particles and a pair of side surface portions (210; 310) provided in both sides of the bottom portion (201; 301) with respect to a circumferential direction of said developing sleeve (103; 103A), characterized in that in a cross-section perpendicular to the bottom portion (201; 301), said side surface portions (210; 310) and a rotational axis of said developing sleeve (103; 103A), each of said grooves (200; 300) is formed so that one particle of the carrier particles with a volume-average particle size cannot contact said side surface portions (210; 310) simultaneously when the one particle of the carrier particles contacts said bottom portion (201; 301) and so that two particles of the carrier particles with the volume-average particle size cannot contact said bottom portion (201; 301) simultaneously and satisfies the following relationships: 2 × r < L, and r / 2 ≤ s < 2 r, where r is the volume-average particle size of the carrier particles, L is a width between said side surface portions (210; 310) at the surface of said developing sleeve (103; 103A) in the cross-section perpendicular to the rotational axis of said developing sleeve (103; 103A), and s is a depth of each of said grooves (200; 300). a developing container accommodating a developer containing toner and carrier particles; a cylindrical developing sleeve (103; 103A) rotatable while carrying the developer in said developing container; a magnet provided in said developing sleeve (103; 103A) and configured to generate a magnetic force for holding the developer; and a plurality of grooves (200; 300) provided at a developer carrying surface of said developing sleeve (103; 103A) and formed along a direction crossing a circumferential direction of said developing sleeve (103; 103A), wherein each of said grooves (200; 300) is formed by a bottom portion (201; 301) contacting the carrier particles and a pair of side surface portions (210; 310) provided in both sides of the bottom portion (201; 301) with respect to a circumferential direction of said developing sleeve (103; 103A), characterized in that in a cross-section perpendicular to the bottom portion (201; 301), said side surface portions (210; 310) and a rotational axis of said developing sleeve (103; 103A), each of said grooves (200; 300) is formed so that one particle of the carrier particles with a volume-average particle size cannot contact said side surface portions (210; 310) simultaneously when the one particle of the carrier particles contacts said bottom portion (201; 301) and so that two particles of the carrier particles with the volume-average particle size cannot contact said bottom portion (201; 301) simultaneously and satisfies the following relationships: 2 × r < L, and r / 2 ≤ s < 2 r, where r is the volume-average particle size of the carrier particles, L is a width between said side surface portions (210; 310) at the surface of said developing sleeve (103; 103A) in the cross-section perpendicular to the rotational axis of said developing sleeve (103; 103A), and s is a depth of each of said grooves (200; 300).