A probe which is to be attached to a finger of a subject to acquire physiological parameter of the subject includes a light emitter that emits a light beam toward the finger, a light detector that outputs a signal corresponding to a light beam that is transmitted through or reflected from the finger, a first holding face that has a portion which is faced with a nail of the finger and another portion which is faced with a side face of the finger, and a second holding face that has a portion which is faced with a pad of the finger and another portion which is faced with the side face of the finger. In a state where the probe is attached to the finger, the first holding face and the second holding face are placed to be continuous to each other.
