A speed reducer angular transmission error identification system comprising: a robot arm including a plurality of joints including a first joint and a second joint; a first joint drive unit including a first speed reducer including an output shaft connected to the first joint, and a first motor which includes an output shaft connected to an input shaft of the first speed reducer and is configured to turn the first joint via the first speed reducer; a first joint control unit that is configured to control an operation of the first motor; a second joint drive unit including a second motor which includes an output shaft connected to the second joint and is configured to turn the second joint; a second joint control unit that is configured to control an operation of the second motor; characterised by a variation data acquisition unit that is configured to acquire first variation data and second variation data associated with each other, the first variation data being data indicating a periodic variation of an operation of the second joint caused by an angular transmission error of the first motor when the first joint control unit controls so as to rotate the output shaft of the first motor in a first direction at a constant first target speed and to turn the first joint from a position of one end to a position of the other end in a predetermined region in a turning range of the first joint and simultaneously the second joint drive unit rotates the output shaft of the second motor at the constant second target speed, the second variation data being data indicating a periodic variation of an operation of the second joint caused by an angular transmission error of the first motor when the first joint control unit controls so as to rotate the output shaft of the first motor in a second direction which is a rotation direction opposite to the first direction at the constant first target speed and to turn the first joint from the position of the other end to the position of the one end in the region and simultaneously the second joint drive unit rotates the output shaft of the second motor at the constant second target speed; a periodic function calculation unit that is configured to calculate a first periodic function that models the first variation data and a second periodic function that models the second variation data; a phase calculation unit that is configured to calculate an average phase which is an average value of a phase of the first periodic function and a phase of the second periodic function; and an angular transmission error identification unit that is configured to calculate a periodic variation of an angular transmission error of the first speed reducer based on the average phase. a robot arm including a plurality of joints including a first joint and a second joint; a first joint drive unit including a first speed reducer including an output shaft connected to the first joint, and a first motor which includes an output shaft connected to an input shaft of the first speed reducer and is configured to turn the first joint via the first speed reducer; a first joint control unit that is configured to control an operation of the first motor; a second joint drive unit including a second motor which includes an output shaft connected to the second joint and is configured to turn the second joint; a second joint control unit that is configured to control an operation of the second motor; characterised by a variation data acquisition unit that is configured to acquire first variation data and second variation data associated with each other, the first variation data being data indicating a periodic variation of an operation of the second joint caused by an angular transmission error of the first motor when the first joint control unit controls so as to rotate the output shaft of the first motor in a first direction at a constant first target speed and to turn the first joint from a position of one end to a position of the other end in a predetermined region in a turning range of the first joint and simultaneously the second joint drive unit rotates the output shaft of the second motor at the constant second target speed, the second variation data being data indicating a periodic variation of an operation of the second joint caused by an angular transmission error of the first motor when the first joint control unit controls so as to rotate the output shaft of the first motor in a second direction which is a rotation direction opposite to the first direction at the constant first target speed and to turn the first joint from the position of the other end to the position of the one end in the region and simultaneously the second joint drive unit rotates the output shaft of the second motor at the constant second target speed; a periodic function calculation unit that is configured to calculate a first periodic function that models the first variation data and a second periodic function that models the second variation data; a phase calculation unit that is configured to calculate an average phase which is an average value of a phase of the first periodic function and a phase of the second periodic function; and an angular transmission error identification unit that is configured to calculate a periodic variation of an angular transmission error of the first speed reducer based on the average phase.