A body motion detector comprising: a body casing having an attachment surface configured to be attached to a waist of a user being an object of body motion detection, the attachment surface being configured to be attached to the waist of the user in a first attachment state in which the attachment surface is arranged parallel to a perpendicular line and in a second attachment state in which the attachment surface is arranged in an inclined manner at a predetermined angle δ with respect to the perpendicular line; a first body motion detecting unit (150; 150A) fixed in the body casing and configured to detect a body motion of the object, the first body motion detecting unit (150; 150A) having a detecting axis (SA; SA1) within a detectable angle range α, the detectable angle range existing by the same extent on both sides with the detecting axis in between, and a sensitivity of the first body motion detecting unit (150; 150A) being a maximum when the body motion occurs along the detecting axis (SA; SA1), characterized in that the detecting axis of the first body motion detecting unit (150; 150A) is inclined at a first inclination angle β from the attachment surface of the body casing such that, in the first attachment state, the first inclination angle β is within the detectable angle range α so that the direction of the perpendicular line exists in the detectable angle range α and, in the second attachment state which fulfills the condition (β-α)≤δ≤(β+α), the detecting axis (SA; SA1) intersects the perpendicular line at an angle β-δ so that the direction of the perpendicular line exists in the detectable angle range α, the first body motion detecting unit (150; 150A) includes a plate shaped member (151; 151a) with a beam part (153; 153a) that bends with the body motion, and a piezoelectric element (156; 156a) attached to a main surface of the beam part (153; 153a) of the plate shaped member (151; 151a), and the first body motion detecting unit (150; 150A) is attached to a wiring substrate accommodated and fixed inside the body casing, the main surface of the beam part (153; 153a) is inclined at the first inclination angle β from a normal of a main surface of the wiring substrate, and the main surface of the wiring substrate is parallel to the attachment surface of the body casing. a body casing having an attachment surface configured to be attached to a waist of a user being an object of body motion detection, the attachment surface being configured to be attached to the waist of the user in a first attachment state in which the attachment surface is arranged parallel to a perpendicular line and in a second attachment state in which the attachment surface is arranged in an inclined manner at a predetermined angle δ with respect to the perpendicular line; a first body motion detecting unit (150; 150A) fixed in the body casing and configured to detect a body motion of the object, the first body motion detecting unit (150; 150A) having a detecting axis (SA; SA1) within a detectable angle range α, the detectable angle range existing by the same extent on both sides with the detecting axis in between, and a sensitivity of the first body motion detecting unit (150; 150A) being a maximum when the body motion occurs along the detecting axis (SA; SA1), characterized in that the detecting axis of the first body motion detecting unit (150; 150A) is inclined at a first inclination angle β from the attachment surface of the body casing such that, in the first attachment state, the first inclination angle β is within the detectable angle range α so that the direction of the perpendicular line exists in the detectable angle range α and, in the second attachment state which fulfills the condition (β-α)≤δ≤(β+α), the detecting axis (SA; SA1) intersects the perpendicular line at an angle β-δ so that the direction of the perpendicular line exists in the detectable angle range α, the first body motion detecting unit (150; 150A) includes a plate shaped member (151; 151a) with a beam part (153; 153a) that bends with the body motion, and a piezoelectric element (156; 156a) attached to a main surface of the beam part (153; 153a) of the plate shaped member (151; 151a), and the first body motion detecting unit (150; 150A) is attached to a wiring substrate accommodated and fixed inside the body casing, the main surface of the beam part (153; 153a) is inclined at the first inclination angle β from a normal of a main surface of the wiring substrate, and the main surface of the wiring substrate is parallel to the attachment surface of the body casing.