A reduction gear comprising: a case of which internal teeth are formed on an inner peripheral side; a gear having external teeth that engage with the internal teeth; a carrier that rotatably holds the gear; an output shaft connected to the carrier; and a pair of main bearings (41, 42) that includes a first main bearing mounted to a portion of the output shaft on the carrier side and a second main bearing mounted to a portion of the output shaft on a side opposite to the carrier, and rotatably holds the output shaft relative to the case, characterized in that: the first main bearing is sandwiched and held between the carrier and a first supporting part which is formed in the case so as to support a portion of the first main bearing on the side opposite to the carrier, the second main bearing is sandwiched and held between an output shaft side supporting part formed in the output shaft so as to support a portion of the second main bearing on the side opposite to the carrier and a second supporting part formed in the case so as to support a portion of the second main bearing on the carrier side, and the reduction gear further comprises: a fixing member arranged in a portion of the carrier on a side opposite to the first main bearing and fixed to the output shaft in a state in which the fixing member is pressed against the carrier side; and an output shaft side spline part formed so as to extend in an axial direction of the output shaft to connect with a carrier side spline part formed in the carrier in such manner that an end part of the output shaft side spline part on the first main bearing side extends to at least the first main bearing. a case of which internal teeth are formed on an inner peripheral side; a gear having external teeth that engage with the internal teeth; a carrier that rotatably holds the gear; an output shaft connected to the carrier; and a pair of main bearings (41, 42) that includes a first main bearing mounted to a portion of the output shaft on the carrier side and a second main bearing mounted to a portion of the output shaft on a side opposite to the carrier, and rotatably holds the output shaft relative to the case, characterized in that: the first main bearing is sandwiched and held between the carrier and a first supporting part which is formed in the case so as to support a portion of the first main bearing on the side opposite to the carrier, the second main bearing is sandwiched and held between an output shaft side supporting part formed in the output shaft so as to support a portion of the second main bearing on the side opposite to the carrier and a second supporting part formed in the case so as to support a portion of the second main bearing on the carrier side, and the reduction gear further comprises: a fixing member arranged in a portion of the carrier on a side opposite to the first main bearing and fixed to the output shaft in a state in which the fixing member is pressed against the carrier side; and an output shaft side spline part formed so as to extend in an axial direction of the output shaft to connect with a carrier side spline part formed in the carrier in such manner that an end part of the output shaft side spline part on the first main bearing side extends to at least the first main bearing. the first main bearing is sandwiched and held between the carrier and a first supporting part which is formed in the case so as to support a portion of the first main bearing on the side opposite to the carrier, the second main bearing is sandwiched and held between an output shaft side supporting part formed in the output shaft so as to support a portion of the second main bearing on the side opposite to the carrier and a second supporting part formed in the case so as to support a portion of the second main bearing on the carrier side, and the reduction gear further comprises: a fixing member arranged in a portion of the carrier on a side opposite to the first main bearing and fixed to the output shaft in a state in which the fixing member is pressed against the carrier side; and an output shaft side spline part formed so as to extend in an axial direction of the output shaft to connect with a carrier side spline part formed in the carrier in such manner that an end part of the output shaft side spline part on the first main bearing side extends to at least the first main bearing. a fixing member arranged in a portion of the carrier on a side opposite to the first main bearing and fixed to the output shaft in a state in which the fixing member is pressed against the carrier side; and an output shaft side spline part formed so as to extend in an axial direction of the output shaft to connect with a carrier side spline part formed in the carrier in such manner that an end part of the output shaft side spline part on the first main bearing side extends to at least the first main bearing.