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1. A webbing take-up device comprising:
a spool having a longitudinal base end of a webbing belt locked thereto and taking up the webbing belt by rotation of the spool in a take-up direction; and a locking unit having a locking unit main body that is operated by rotation of a first rotating body in a predetermined direction so as to regulate rotation of the spool in a pull-out direction that is opposite to the take-up direction, and having a rotational acceleration detecting unit that is operated by rotation of the spool in the pull-out direction at an acceleration of greater than or equal to a predetermined value so as to rotate the first rotating body in the predetermined direction, wherein the rotational acceleration detecting unit comprises: a second rotating body that rotates in the predetermined direction due to rotation of the spool in the pull-out direction and that is provided with a retaining portion; a rotation transmission member provided at the second rotating body so as to be capable of being brought closer to and further separated from the first rotating body, and transmitting the rotation of the second rotating body in the predetermined direction to the first rotating body in a state in which the rotation transmission member is engaged with the first rotating body; an inertial mass body having a pressing portion and provided at the second rotating body, and rotating in a direction opposite to the predetermined direction relative to the second rotating body, whereby the pressing portion presses the rotation transmission member to make the rotation transmission member engage with the first rotating body; and a biasing means provided at the second rotating body to bias the inertial mass body in the predetermined direction, whereby the inertial mass body pressure contacts the rotation transmission member and biases the rotation transmission member via a biasing force so as to move apart from the first rotating body, and the biasing means further allowing relative rotation to be produced between the second rotating body and the inertial mass body against the biasing force when the second rotating body has rotated in the predetermined direction at an acceleration greater than or equal to the predetermined value; wherein the retaining portion pinches the rotation transmission member together with the inertial mass body biased by the biasing means, thereby retaining the rotation transmission member in a state in which the rotation transmission member does not engage with the first rotating body, and to release retention of the rotation transmission member when relative rotation is produced between the second rotating body and the inertial mass body, and wherein a projection is formed on the rotation transmission member that projects toward the second rotating body along the direction of a rotation axis of the second rotating body, the second rotating body is formed with a hole portion into which the projection enters and is pressure contacted by an inner peripheral portion thereof as the rotation transmission member is pressed by the inertial mass body by the biasing force of the biasing means, and the inner peripheral portion of the hole portion serves as the retaining portion, wherein the projection is formed at one end portion in a length direction of the rotation transmission member.
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