Provided is a degradation diagnostic device, which is configured to perform a diagnosis for degradation of a compressor including a compression mechanism unit to be driven along with a rotational motion of a rotary shaft, and a shell, which is configured to form an outer casing. The degradation diagnostic device includes a vibration detection device to be fixed onto an outer wall of the shell at a position at which the compression mechanism unit is located. The vibration detection device includes: a vibration sensor, which is configured to detect vibration of the compressor; a sensor holding unit including the vibration sensor; a base portion to which the sensor holding unit is connected; and a plurality of projecting portions, which are formed on a surface of the base portion on a side opposite to the sensor holding unit and to be brought into contact with the outer wall.