An active noise reduction device includes: a reference signal input terminal that receives a reference signal outputted by a reference signal source and having a correlation with noise in a space in an automobile, the reference signal source being attached to the automobile; a test signal source that outputs a test signal to a loudspeaker attached to the automobile, the loudspeaker being used to output a canceling sound for reducing the noise; and an anomaly determiner that determines whether the reference signal source has an anomaly, based on the reference signal inputted from the reference signal source to the reference signal input terminal when the test signal is outputted to the loudspeaker.