An information processing device comprising: an acquisition unit configured to acquire an image captured by an imaging unit and a detection result of a gravitational acceleration by a detection unit supported by the imaging unit, for each of a plurality of viewpoints different from one another; an image processing unit configured to extract a plurality of line segments extending in a gravity direction from the image and specify an intersection of straight lines obtained by respectively extending the plurality of line segments on a basis of a subject captured in the image for each of the plurality of viewpoints; and a calculation unit configured to: calculate a vector in the gravity direction in a first coordinate system associated with the imaging unit at a viewpoint on a basis of a specification result of the intersection for each of the plurality of viewpoints; and calculate a function for converting a posture in one coordinate system into a posture in another coordinate system between the first coordinate system associated with the imaging unit and a second coordinate system associated with the detection unit on a basis of the vector in the gravity direction in the first coordinate system and a vector according to the gravitational acceleration in the second coordinate system according to one of the following mathematical equations: R = argmin R ∑ n = 1 ⋯ N c n c n − R a n a n R, b, S = argmin R, b, S ∑ n = 1 ⋯ N g c n c n − RS a n − b with R denoting the function for converting the posture in the one coordinate system into the posture in the other coordinate system, c n denoting the respective vector in the gravity direction for the plurality of viewpoints, a n denoting the respective vector according to the gravitational acceleration for the plurality of viewpoints, g denoting the magnitude of the gravitational acceleration, S denoting a matrix representing a scale factor of the detection unit, and b denoting a vector representing a bias of the detection unit. an acquisition unit configured to acquire an image captured by an imaging unit and a detection result of a gravitational acceleration by a detection unit supported by the imaging unit, for each of a plurality of viewpoints different from one another; an image processing unit configured to extract a plurality of line segments extending in a gravity direction from the image and specify an intersection of straight lines obtained by respectively extending the plurality of line segments on a basis of a subject captured in the image for each of the plurality of viewpoints; and a calculation unit configured to: calculate a vector in the gravity direction in a first coordinate system associated with the imaging unit at a viewpoint on a basis of a specification result of the intersection for each of the plurality of viewpoints; and calculate a function for converting a posture in one coordinate system into a posture in another coordinate system between the first coordinate system associated with the imaging unit and a second coordinate system associated with the detection unit on a basis of the vector in the gravity direction in the first coordinate system and a vector according to the gravitational acceleration in the second coordinate system according to one of the following mathematical equations: R = argmin R ∑ n = 1 ⋯ N c n c n − R a n a n R, b, S = argmin R, b, S ∑ n = 1 ⋯ N g c n c n − RS a n − b with R denoting the function for converting the posture in the one coordinate system into the posture in the other coordinate system, c n denoting the respective vector in the gravity direction for the plurality of viewpoints, a n denoting the respective vector according to the gravitational acceleration for the plurality of viewpoints, g denoting the magnitude of the gravitational acceleration, S denoting a matrix representing a scale factor of the detection unit, and b denoting a vector representing a bias of the detection unit. calculate a vector in the gravity direction in a first coordinate system associated with the imaging unit at a viewpoint on a basis of a specification result of the intersection for each of the plurality of viewpoints; and calculate a function for converting a posture in one coordinate system into a posture in another coordinate system between the first coordinate system associated with the imaging unit and a second coordinate system associated with the detection unit on a basis of the vector in the gravity direction in the first coordinate system and a vector according to the gravitational acceleration in the second coordinate system according to one of the following mathematical equations: R = argmin R ∑ n = 1 ⋯ N c n c n − R a n a n R, b, S = argmin R, b, S ∑ n = 1 ⋯ N g c n c n − RS a n − b with R denoting the function for converting the posture in the one coordinate system into the posture in the other coordinate system, c n denoting the respective vector in the gravity direction for the plurality of viewpoints, a n denoting the respective vector according to the gravitational acceleration for the plurality of viewpoints, g denoting the magnitude of the gravitational acceleration, S denoting a matrix representing a scale factor of the detection unit, and b denoting a vector representing a bias of the detection unit.