An internal engagement transmission mechanism, comprising: an outer wheel, wherein a first number of outer wheel internal teeth are provided on an inner edge of the outer wheel, each of the outer wheel internal teeth comprises an outer wheel internal tooth top and outer wheel internal tooth waists (314, 316) symmetrical with respect to the outer wheel internal tooth top, each of the outer wheel internal tooth waists (314, 316) comprises an engagement portion (336, 346); and an inner wheel (121, 122, 123, 124), wherein a second number of inner wheel external teeth are provided on an outer edge of the inner wheel (121, 122, 123, 124), each of the inner wheel external teeth comprises an inner wheel external tooth top and inner wheel external tooth waists (414,416) symmetrical with respect to the inner wheel external tooth top, each of the inner wheel external tooth waists (414,416) comprises an engagement portion(436, 446), the first number being greater than the second number; wherein the inner wheel (121, 122, 123, 124) is arranged inside the outer wheel and eccentrically rotates with respect to the outer wheel, the inner wheel (121, 122, 123, 124) and the outer wheel form internal engagement transmission via the engagement of the engagement portion (436, 446) of the inner wheel external tooth waist (414,416) and the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316); characterized in that the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316) and the engagement portion (436, 446) of the inner wheel external tooth waist (414,416) are curved surfaces which have the same shape and are overlapped, and the outer wheel internal tooth waist (314, 316) and the inner wheel external tooth waist (414,416) are designed such that when the outer wheel and the inner wheel (121, 122, 123, 124) are in engagement transmission, at least one inner wheel external tooth in the inner wheel external teeth is capable of engaging with the outer wheel internal teeth, and for each of the inner wheel external teeth engaged with the outer wheel internal teeth, as the inner wheel eccentrically rotates, the engagement portion (436, 446) of at least one of the inner wheel external tooth waists (414,416) is capable of forming surface-contact engagement with the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316), and the engagement portion(436, 446) of the at least one of the inner wheel external tooth waists (414, 416) and the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316) are overlapped in a fully engaged state. an outer wheel, wherein a first number of outer wheel internal teeth are provided on an inner edge of the outer wheel, each of the outer wheel internal teeth comprises an outer wheel internal tooth top and outer wheel internal tooth waists (314, 316) symmetrical with respect to the outer wheel internal tooth top, each of the outer wheel internal tooth waists (314, 316) comprises an engagement portion (336, 346); and an inner wheel (121, 122, 123, 124), wherein a second number of inner wheel external teeth are provided on an outer edge of the inner wheel (121, 122, 123, 124), each of the inner wheel external teeth comprises an inner wheel external tooth top and inner wheel external tooth waists (414,416) symmetrical with respect to the inner wheel external tooth top, each of the inner wheel external tooth waists (414,416) comprises an engagement portion(436, 446), the first number being greater than the second number; wherein the inner wheel (121, 122, 123, 124) is arranged inside the outer wheel and eccentrically rotates with respect to the outer wheel, the inner wheel (121, 122, 123, 124) and the outer wheel form internal engagement transmission via the engagement of the engagement portion (436, 446) of the inner wheel external tooth waist (414,416) and the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316); characterized in that the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316) and the engagement portion (436, 446) of the inner wheel external tooth waist (414,416) are curved surfaces which have the same shape and are overlapped, and the outer wheel internal tooth waist (314, 316) and the inner wheel external tooth waist (414,416) are designed such that when the outer wheel and the inner wheel (121, 122, 123, 124) are in engagement transmission, at least one inner wheel external tooth in the inner wheel external teeth is capable of engaging with the outer wheel internal teeth, and for each of the inner wheel external teeth engaged with the outer wheel internal teeth, as the inner wheel eccentrically rotates, the engagement portion (436, 446) of at least one of the inner wheel external tooth waists (414,416) is capable of forming surface-contact engagement with the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316), and the engagement portion(436, 446) of the at least one of the inner wheel external tooth waists (414, 416) and the engagement portion (336, 346) of the outer wheel internal tooth waist (314, 316) are overlapped in a fully engaged state.