A probe configured to be attached to a finger of a subject to acquire physiological parameter of the subject comprising: a light emitter configured to emit a light beam toward the finger; a light detector configured to output a signal corresponding to a light beam that is transmitted through or reflected from the finger; a first holding face (12a, 13a) 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 (12b, 13b) 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, wherein, in a state where the probe is attached to the finger, the first holding face (12a, 13a) and the second holding face (12b, 13b) are placed so as to be continuous to each other in the direction from nail to pad of the finger, wherein a curvature of a sectional shape of the first holding face (12a, 13a) is different from that of the second holding face (12b, 13b), and wherein the probe further comprises: a left side portion that is placed to be faceable with a left side face of the finger; a right side portion that is placed to be faceable with a right side face of the finger; and a hinge portion configured to connect the left side portion and the right side portion together in a manner that a gap between the left side portion and the right side portion is adjustable, wherein the light emitter is placed in one of the left side portion and the right side portion, the light detector is placed in the other one of the left side portion and the right side portion, and each of the left side portion and the right side portion has the first holding face (12a, 13a) and the second holding face (12b, 13b). a light emitter configured to emit a light beam toward the finger; a light detector configured to output a signal corresponding to a light beam that is transmitted through or reflected from the finger; a first holding face (12a, 13a) 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 (12b, 13b) 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, wherein, in a state where the probe is attached to the finger, the first holding face (12a, 13a) and the second holding face (12b, 13b) are placed so as to be continuous to each other in the direction from nail to pad of the finger, wherein a curvature of a sectional shape of the first holding face (12a, 13a) is different from that of the second holding face (12b, 13b), and wherein the probe further comprises: a left side portion that is placed to be faceable with a left side face of the finger; a right side portion that is placed to be faceable with a right side face of the finger; and a hinge portion configured to connect the left side portion and the right side portion together in a manner that a gap between the left side portion and the right side portion is adjustable, wherein the light emitter is placed in one of the left side portion and the right side portion, the light detector is placed in the other one of the left side portion and the right side portion, and each of the left side portion and the right side portion has the first holding face (12a, 13a) and the second holding face (12b, 13b). a left side portion that is placed to be faceable with a left side face of the finger; a right side portion that is placed to be faceable with a right side face of the finger; and a hinge portion configured to connect the left side portion and the right side portion together in a manner that a gap between the left side portion and the right side portion is adjustable, wherein the light emitter is placed in one of the left side portion and the right side portion, the light detector is placed in the other one of the left side portion and the right side portion, and each of the left side portion and the right side portion has the first holding face (12a, 13a) and the second holding face (12b, 13b).