A diagnosis device, comprising: a learning information acquisition unit configured to acquire learning information including a learning value of a first device mounted in at least one first vehicle (1-2, 1-3) and reliability information indicating the reliability of the learning value of the first device, wherein the at least one first vehicle (1-2, 1-3) is present in a predetermined range from a second vehicle; a learning unit configured to calculate a learning value of a second device mounted in the second vehicle using the learning value of the first device and the reliability of the learning value of the first device; and a diagnosis unit configured to diagnose an operation state of the second device of the second vehicle by comparing a detection value of the second device of the second vehicle with the learning value of the second device calculated by the learning unit, the detection value of the second device being a value sensed by the second device or a control value for the second device, characterized in that the learning information acquisition unit is configured to acquire the learning value of the first device as a pair of a detection value of the first device and an error value of the first device, the detection value of the first device being a value sensed by the first device or a control value for the first device and the error value being a difference between the detection value of the first device and the learning value of the first device; and the learning unit is configured to calculate the learning value of the second device using the learning value of the first device in which the error value is equal to or smaller than a threshold value. a learning information acquisition unit configured to acquire learning information including a learning value of a first device mounted in at least one first vehicle (1-2, 1-3) and reliability information indicating the reliability of the learning value of the first device, wherein the at least one first vehicle (1-2, 1-3) is present in a predetermined range from a second vehicle; a learning unit configured to calculate a learning value of a second device mounted in the second vehicle using the learning value of the first device and the reliability of the learning value of the first device; and a diagnosis unit configured to diagnose an operation state of the second device of the second vehicle by comparing a detection value of the second device of the second vehicle with the learning value of the second device calculated by the learning unit, the detection value of the second device being a value sensed by the second device or a control value for the second device, characterized in that the learning information acquisition unit is configured to acquire the learning value of the first device as a pair of a detection value of the first device and an error value of the first device, the detection value of the first device being a value sensed by the first device or a control value for the first device and the error value being a difference between the detection value of the first device and the learning value of the first device; and the learning unit is configured to calculate the learning value of the second device using the learning value of the first device in which the error value is equal to or smaller than a threshold value.