An image generating device, comprising: a captured image inputting module configured to input an image captured by an imaging device mounted on a water-surface movable body; a position acquiring module configured to acquire positional information indicative of a position of the water-surface movable body; a posture acquiring module configured to acquire posture information indicative of a posture of the water-surface movable body; an additional display information acquiring module configured to non-continuously acquire additional display information including information indicative of a position of a target object at consecutive points in time; a synthesized image generating module configured to generate, every time additional display information is acquired, a synthesized image where a graphic rendering a three-dimensional virtual reality object indicative of the additional display information is synthesized with the captured image based on measured position and imaging direction of the imaging device relative to the water-surface movable body, the positional information, the posture information, and the additional display information; a target object position estimating module configured to estimate a position of the target object at a timepoint between acquisition of two consecutive pieces of additional display information, based on a speed of the target object included in the last acquired additional display information; and an image updating module configured to cause the synthesized image generating module to update the rendering of the three-dimensional virtual reality object of the target object in the synthesized image such that the three-dimensional virtual reality object is rearranged in the synthesized image based on a result of the estimation by the target object position estimating module at said timepoint between acquisition of two consecutive pieces of additional display information. a captured image inputting module configured to input an image captured by an imaging device mounted on a water-surface movable body; a position acquiring module configured to acquire positional information indicative of a position of the water-surface movable body; a posture acquiring module configured to acquire posture information indicative of a posture of the water-surface movable body; an additional display information acquiring module configured to non-continuously acquire additional display information including information indicative of a position of a target object at consecutive points in time; a synthesized image generating module configured to generate, every time additional display information is acquired, a synthesized image where a graphic rendering a three-dimensional virtual reality object indicative of the additional display information is synthesized with the captured image based on measured position and imaging direction of the imaging device relative to the water-surface movable body, the positional information, the posture information, and the additional display information; a target object position estimating module configured to estimate a position of the target object at a timepoint between acquisition of two consecutive pieces of additional display information, based on a speed of the target object included in the last acquired additional display information; and an image updating module configured to cause the synthesized image generating module to update the rendering of the three-dimensional virtual reality object of the target object in the synthesized image such that the three-dimensional virtual reality object is rearranged in the synthesized image based on a result of the estimation by the target object position estimating module at said timepoint between acquisition of two consecutive pieces of additional display information.