A maintenance information output system is provided that includes an iron amount detecting device, a receiver, a maintenance information output apparatus, and a controlling portion. The iron amount detecting device is attached to a speed reducer, and detects an amount of iron in a lubricant in the speed reducer. The receiver receives data of the amount of iron detected by the iron amount detecting device. The maintenance information output apparatus has an outputting portion which outputs maintenance information of the speed reducer. The maintenance information output apparatus has a receiving portion which receives the data from the receiver. When the amount of iron is equal to or larger than a threshold value, the controlling portion causes the outputting portion to output the maintenance information.