A vehicle roll angle estimation system to be mounted on a vehicle which is inclinable to a left-right direction with respect to a traveling direction, comprising: an imaging device capable of capturing forward or rearward images from the vehicle; and a roll angle estimation device configured to obtain a plurality of images including a target image captured in series by the imaging device and analyze the image, thereby estimating a roll angle of the imaging device which changes as the vehicle's inclination changes; wherein the roll angle estimation device includes: a histogram generator configured to generate of a brightness gradient orientation histogram which shows a frequency distribution of brightness gradient orientation of a plurality of pixels contained in the image; an estimation section configured to calculate a skewness which indicates a symmetricalness of the brightness gradient orientation histogram by using θi that represents a value of brightness gradient orientation in a plurality of pixels contained in an image, θμ that represents an average value of the brightness gradient orientation in the plurality of pixels, n that represents a quantity of pixels for which the brightness gradient orientation was calculated, and s that represents a standard deviation of brightness gradient orientation in the plurality of pixels, and estimating the roll angle based on the calculated skewness; the estimation section is configured to estimate that the roll angle is zero degrees if the skewness is within a predetermined range with respect to zero, and, to select a peak which represents a peak of horizontal-direction edges or vertical-direction edges in a brightness gradient orientation histogram of a past image that was estimated as having the roll angle of zero degrees, from the brightness gradient orientation histogram of the target image and to estimate a roll angle in the target image based on a brightness gradient orientation of the selected peak, if the target image does not have the skewness having a value within the predetermined range with respect to zero. an imaging device capable of capturing forward or rearward images from the vehicle; and a roll angle estimation device configured to obtain a plurality of images including a target image captured in series by the imaging device and analyze the image, thereby estimating a roll angle of the imaging device which changes as the vehicle's inclination changes; wherein the roll angle estimation device includes: a histogram generator configured to generate of a brightness gradient orientation histogram which shows a frequency distribution of brightness gradient orientation of a plurality of pixels contained in the image; an estimation section configured to calculate a skewness which indicates a symmetricalness of the brightness gradient orientation histogram by using θi that represents a value of brightness gradient orientation in a plurality of pixels contained in an image, θμ that represents an average value of the brightness gradient orientation in the plurality of pixels, n that represents a quantity of pixels for which the brightness gradient orientation was calculated, and s that represents a standard deviation of brightness gradient orientation in the plurality of pixels, and estimating the roll angle based on the calculated skewness; the estimation section is configured to estimate that the roll angle is zero degrees if the skewness is within a predetermined range with respect to zero, and, to select a peak which represents a peak of horizontal-direction edges or vertical-direction edges in a brightness gradient orientation histogram of a past image that was estimated as having the roll angle of zero degrees, from the brightness gradient orientation histogram of the target image and to estimate a roll angle in the target image based on a brightness gradient orientation of the selected peak, if the target image does not have the skewness having a value within the predetermined range with respect to zero. a histogram generator configured to generate of a brightness gradient orientation histogram which shows a frequency distribution of brightness gradient orientation of a plurality of pixels contained in the image; an estimation section configured to calculate a skewness which indicates a symmetricalness of the brightness gradient orientation histogram by using θi that represents a value of brightness gradient orientation in a plurality of pixels contained in an image, θμ that represents an average value of the brightness gradient orientation in the plurality of pixels, n that represents a quantity of pixels for which the brightness gradient orientation was calculated, and s that represents a standard deviation of brightness gradient orientation in the plurality of pixels, and estimating the roll angle based on the calculated skewness; the estimation section is configured to estimate that the roll angle is zero degrees if the skewness is within a predetermined range with respect to zero, and, to select a peak which represents a peak of horizontal-direction edges or vertical-direction edges in a brightness gradient orientation histogram of a past image that was estimated as having the roll angle of zero degrees, from the brightness gradient orientation histogram of the target image and to estimate a roll angle in the target image based on a brightness gradient orientation of the selected peak, if the target image does not have the skewness having a value within the predetermined range with respect to zero.