A drive transmission unit comprising a planetary gear mechanism, the planetary gear mechanism comprising: a gear mount; an outer gear fixed to the gear mount; a first sun gear disposed on a rotary shaft and rotated by a power source, and a second sun gear, the first sun gear being coaxially disposed on the same shaft as the outer gear; a plurality of first planetary gears to engage the first sun gear and the outer gear and a plurality of second planetary gears to engage the second sun gear and the outer gear, the plurality of first and second planetary gears (15, 18) disposed along an inner circumferential surface of the outer gear; a first carrier to rotatably support the first planetary gear and a second carrier to rotatably support the second planetary gear, the first carrier is rotatable on the same shaft as the first sun gear and the outer gear, wherein the first carrier contacts the first sun gear, and the second carrier contacts the second sun gear as the first sun gear and the second sun gear move in a direction of thrust due to engagement with the plurality of first and second planetary gears (15, 18), wherein the outer gear, the sun gears (12, 17), and the plurality of first and second planetary gears (15, 18) are helical gears, characterized in that a first grease retaining portion to retain grease near a first contact portion at which the first sun gear contacts the first carrier as the first sun gear moves in the direction of thrust due to engagement with the plurality of first planetary gears and a second grease retaining portion to retain grease near a second contact portion at which the second sun gear contacts the second carrier as the second sun gear moves in the direction of thrust due to engagement with the plurality second of planetary gears, wherein the grease retaining portions (H1, H2) are formed around the first contact portion at which the first carrier and the first sun gear and around the second contact portion at which the second carrier and the second gear contacts respectively, wherein one of the sun gears (12, 17) and the carriers (16, 19) include a cylindrical concavity (16a, 19a), and the other one of which includes a projection (12a, 17a) that contacts the center of the concavity (16a, 19a), wherein the grease retaining portion is a space between the projection (12a, 17a) and the concave portion (16a, 19a). a gear mount; an outer gear fixed to the gear mount; a first sun gear disposed on a rotary shaft and rotated by a power source, and a second sun gear, the first sun gear being coaxially disposed on the same shaft as the outer gear; a plurality of first planetary gears to engage the first sun gear and the outer gear and a plurality of second planetary gears to engage the second sun gear and the outer gear, the plurality of first and second planetary gears (15, 18) disposed along an inner circumferential surface of the outer gear; a first carrier to rotatably support the first planetary gear and a second carrier to rotatably support the second planetary gear, the first carrier is rotatable on the same shaft as the first sun gear and the outer gear, wherein the first carrier contacts the first sun gear, and the second carrier contacts the second sun gear as the first sun gear and the second sun gear move in a direction of thrust due to engagement with the plurality of first and second planetary gears (15, 18), wherein the outer gear, the sun gears (12, 17), and the plurality of first and second planetary gears (15, 18) are helical gears, characterized in that a first grease retaining portion to retain grease near a first contact portion at which the first sun gear contacts the first carrier as the first sun gear moves in the direction of thrust due to engagement with the plurality of first planetary gears and a second grease retaining portion to retain grease near a second contact portion at which the second sun gear contacts the second carrier as the second sun gear moves in the direction of thrust due to engagement with the plurality second of planetary gears, wherein the grease retaining portions (H1, H2) are formed around the first contact portion at which the first carrier and the first sun gear and around the second contact portion at which the second carrier and the second gear contacts respectively, wherein one of the sun gears (12, 17) and the carriers (16, 19) include a cylindrical concavity (16a, 19a), and the other one of which includes a projection (12a, 17a) that contacts the center of the concavity (16a, 19a), wherein the grease retaining portion is a space between the projection (12a, 17a) and the concave portion (16a, 19a).