A speed reducing device (1, 1') having a power source, comprising: a motor serving as the power source and comprising: a stator portion; and a rotor portion, wherein the rotor portion is driven to rotate by the stator portion, and the rotor portion comprises a rotor casing assembly having a hollow structure, a first eccentric ring and a second eccentric ring, wherein the first eccentric ring and the second eccentric ring are disposed beside each other and disposed on an inner surface of the rotor casing assembly; and a speed reducing mechanism, wherein at least a portion of the speed reducing mechanism is located at an inner side of the motor and accommodated within the rotor portion; characterised in that the speed reducing mechanism comprises a first roller assembly located at a first side of the speed reducing mechanism and comprising a first reducer casing and at least one first roller, wherein the at least one first roller is disposed on the first reducer casing; a second roller assembly located at a second side of the speed reducing mechanism and comprising a second reducer casing and at least one second roller, wherein the at least one second roller is disposed on the second reducer casing; a third roller assembly disposed between the first roller assembly and the second roller assembly, wherein the third roller assembly is accommodated within the rotor portion, the first roller assembly and the second roller assembly, and wherein the third roller assembly comprises an output shaft and at least one third roller, wherein the at least one third roller is mounted on the output shaft; a first cycloid disc set (30, 30') mounted around the output shaft and disposed within the first eccentric ring, and comprising a plurality of first tooth structures (300, 300') and a plurality of second tooth structures; and a second cycloid disc set (31, 31') mounted around the output shaft and disposed within the second eccentric ring, and comprising a plurality of third tooth structures (310, 310') and a plurality of fourth tooth structures, wherein the first roller is contacted with at least one of the plurality of first tooth structures (300, 300'), the second roller is contacted with at least one of the plurality of third tooth structures (310, 310'), and the third roller is contacted with at least one of the second tooth structures and at least one of the fourth tooth structures. a stator portion; and a rotor portion, wherein the rotor portion is driven to rotate by the stator portion, and the rotor portion comprises a rotor casing assembly having a hollow structure, a first eccentric ring and a second eccentric ring, wherein the first eccentric ring and the second eccentric ring are disposed beside each other and disposed on an inner surface of the rotor casing assembly; and a speed reducing mechanism, wherein at least a portion of the speed reducing mechanism is located at an inner side of the motor and accommodated within the rotor portion; characterised in that the speed reducing mechanism comprises a first roller assembly located at a first side of the speed reducing mechanism and comprising a first reducer casing and at least one first roller, wherein the at least one first roller is disposed on the first reducer casing; a second roller assembly located at a second side of the speed reducing mechanism and comprising a second reducer casing and at least one second roller, wherein the at least one second roller is disposed on the second reducer casing; a third roller assembly disposed between the first roller assembly and the second roller assembly, wherein the third roller assembly is accommodated within the rotor portion, the first roller assembly and the second roller assembly, and wherein the third roller assembly comprises an output shaft and at least one third roller, wherein the at least one third roller is mounted on the output shaft; a first cycloid disc set (30, 30') mounted around the output shaft and disposed within the first eccentric ring, and comprising a plurality of first tooth structures (300, 300') and a plurality of second tooth structures; and a second cycloid disc set (31, 31') mounted around the output shaft and disposed within the second eccentric ring, and comprising a plurality of third tooth structures (310, 310') and a plurality of fourth tooth structures, wherein the first roller is contacted with at least one of the plurality of first tooth structures (300, 300'), the second roller is contacted with at least one of the plurality of third tooth structures (310, 310'), and the third roller is contacted with at least one of the second tooth structures and at least one of the fourth tooth structures.