A radar image processing device, comprising: a phase shift component calculating unit for calculating a phase shift component φ(x) of complex image pixels at the position x, in a first axis direction, denoted as x-axis direction, on a two-dimensional inclined surface included in a first radar image and a second radar image, each of the first and second radar image being a radar image showing an intensity distribution of radio waves emitted by a radar, then reflected by an observation area, and received by the radar, a plurality of pixels included in the first radar image and a plurality of pixels included in the second radar image each having a complex pixel value, represented by an amplitude and a phase, the first axis being an axis of the inclined surface inclined with respect to a ground-range direction, a second axis of the inclined surface being an azimuth direction; a phase calculating unit for calculating a phase ρ(z 0) on a parallel surface, the parallel surface being a surface parallel to the inclined surface, and at a distance of z 0 from the inclined surface, with respect to the inclined surface, ρ(z 0) being dependent on the distance z 0 between the inclined surface and the parallel surface, the distance z 0 being a parameter set in advance by a user; and a phase difference calculating unit for calculating a phase difference Δφ(x,z 0) in each of a plurality of reflected signals present in each of the pixels included in the first and second radar images, the phase difference Δφ(x,z 0) being a difference between phases with respect to the respective radio wave receiving points, said phase difference Δφ(x,z 0) being calculated by forming a sum of the phase shift component (φ(x) calculated by the phase shift component calculating unit and the phase ρ(z 0) calculated by the phase calculating unit; a rotation amount calculating unit for calculating each of phase rotation amounts exp[j·Δφ(x,z 0)] in a plurality of pixels included in the second radar image from each phase difference Δφ(x,z 0) calculated by the phase difference calculating unit; and a difference calculating unit for rotating phases in a plurality of pixels included in the second radar image on a basis of the respective rotation amounts exp[j·Δφ(x,z0)] calculated by the rotation amount calculating unit, and calculating a difference between pixel values of pixels being complex numbers at corresponding pixel positions among a plurality of pixels included in the first radar image and a plurality of pixels resulting from phase rotation included in the second radar image. a phase shift component calculating unit for calculating a phase shift component φ(x) of complex image pixels at the position x, in a first axis direction, denoted as x-axis direction, on a two-dimensional inclined surface included in a first radar image and a second radar image, each of the first and second radar image being a radar image showing an intensity distribution of radio waves emitted by a radar, then reflected by an observation area, and received by the radar, a plurality of pixels included in the first radar image and a plurality of pixels included in the second radar image each having a complex pixel value, represented by an amplitude and a phase, the first axis being an axis of the inclined surface inclined with respect to a ground-range direction, a second axis of the inclined surface being an azimuth direction; each of the first and second radar image being a radar image showing an intensity distribution of radio waves emitted by a radar, then reflected by an observation area, and received by the radar, a plurality of pixels included in the first radar image and a plurality of pixels included in the second radar image each having a complex pixel value, represented by an amplitude and a phase, the first axis being an axis of the inclined surface inclined with respect to a ground-range direction, a second axis of the inclined surface being an azimuth direction; a phase calculating unit for calculating a phase ρ(z 0) on a parallel surface, the parallel surface being a surface parallel to the inclined surface, and at a distance of z 0 from the inclined surface, with respect to the inclined surface, ρ(z 0) being dependent on the distance z 0 between the inclined surface and the parallel surface, the distance z 0 being a parameter set in advance by a user; and a phase difference calculating unit for calculating a phase difference Δφ(x,z 0) in each of a plurality of reflected signals present in each of the pixels included in the first and second radar images, the phase difference Δφ(x,z 0) being a difference between phases with respect to the respective radio wave receiving points, said phase difference Δφ(x,z 0) being calculated by forming a sum of the phase shift component (φ(x) calculated by the phase shift component calculating unit and the phase ρ(z 0) calculated by the phase calculating unit; a rotation amount calculating unit for calculating each of phase rotation amounts exp[j·Δφ(x,z 0)] in a plurality of pixels included in the second radar image from each phase difference Δφ(x,z 0) calculated by the phase difference calculating unit; and a difference calculating unit for rotating phases in a plurality of pixels included in the second radar image on a basis of the respective rotation amounts exp[j·Δφ(x,z0)] calculated by the rotation amount calculating unit, and calculating a difference between pixel values of pixels being complex numbers at corresponding pixel positions among a plurality of pixels included in the first radar image and a plurality of pixels resulting from phase rotation included in the second radar image.