An inertial force sensor comprising: a detecting device which is adapted to detect an inertial force, wherein the detecting device has a planar structure and includes: two first orthogonal arms, each of the first orthogonal arms has a first arm and two second arms, the two second arms being connected to a first extremity of the first arm, a supporting portion being connected to a second extremity of each the first arms and adapted to support the first arms such that the first arms extend along a first direction in a symmetric manner from the supporting portion, wherein the first arms and supporting portion are aligned with each other, two fixing arms connected to the supporting portion and fixed to a mounting substrate on which the detecting device is to be mounted, and base portions being fixed to the fixing arms and to the mounting substrate on which detecting device is mounted, wherein each of the second arms of the same first orthogonal arm has a first portion connected at a first extremity of the first portion to the first extremity of the first arm and extending along a second direction orthogonal to the first direction, a second portion connected at a first extremity of the second portion to a second extremity of the first portion and extending along the first direction away from the supporting portion, and a third portion connected at a first extremity of the third portion to a second extremity of the second portion and extending along the second direction toward the first arms, second extremities of the third portions of the second arms of the same first orthogonal arm facing each other, the second arms comprising a weight portion formed at the second extremity of the third portion; the two second arms of the same first orthogonal arm being arranged symmetrically to each other with respect to the first direction, and wherein the two first orthogonal arms are arranged symmetrically to each other with respect to the second direction. a detecting device which is adapted to detect an inertial force, wherein the detecting device has a planar structure and includes: two first orthogonal arms, each of the first orthogonal arms has a first arm and two second arms, the two second arms being connected to a first extremity of the first arm, a supporting portion being connected to a second extremity of each the first arms and adapted to support the first arms such that the first arms extend along a first direction in a symmetric manner from the supporting portion, wherein the first arms and supporting portion are aligned with each other, two fixing arms connected to the supporting portion and fixed to a mounting substrate on which the detecting device is to be mounted, and base portions being fixed to the fixing arms and to the mounting substrate on which detecting device is mounted, two first orthogonal arms, each of the first orthogonal arms has a first arm and two second arms, the two second arms being connected to a first extremity of the first arm, a supporting portion being connected to a second extremity of each the first arms and adapted to support the first arms such that the first arms extend along a first direction in a symmetric manner from the supporting portion, wherein the first arms and supporting portion are aligned with each other, two fixing arms connected to the supporting portion and fixed to a mounting substrate on which the detecting device is to be mounted, and base portions being fixed to the fixing arms and to the mounting substrate on which detecting device is mounted, wherein each of the second arms of the same first orthogonal arm has a first portion connected at a first extremity of the first portion to the first extremity of the first arm and extending along a second direction orthogonal to the first direction, a second portion connected at a first extremity of the second portion to a second extremity of the first portion and extending along the first direction away from the supporting portion, and a third portion connected at a first extremity of the third portion to a second extremity of the second portion and extending along the second direction toward the first arms, second extremities of the third portions of the second arms of the same first orthogonal arm facing each other, the second arms comprising a weight portion formed at the second extremity of the third portion; the two second arms of the same first orthogonal arm being arranged symmetrically to each other with respect to the first direction, and wherein the two first orthogonal arms are arranged symmetrically to each other with respect to the second direction.