A failure detection device includes a formaldehyde sensor that detects the generation of formaldehyde, a vibration sensor that detects vibration of a spindle head, and a machine controller that detects the occurrence of an abnormality of a spindle bearing. The machine controller determines the occurrence of the abnormality of the spindle bearing and a cause of the abnormality based on a change in the output value of the formaldehyde sensor over time and a change in the output value of the vibration sensor over time.