1. An imaging apparatus for diagnosis which reconfigures a three-dimensional image of a blood vessel by using a probe emitting light toward a vascular lumen surface, and accommodating an imaging core for detecting reflection light from the blood vessel, and to move the imaging core along the probe at a predetermined speed while the imaging core is rotated, the apparatus comprising: a processor configured to: derive line data configured to include multiple luminance values arrayed side by side in a radial direction from a rotation center of the imaging core per each rotation angle in a rotation, based on data obtained by the imaging core during a period while the imaging core is rotated and moved; and calculate a two-dimensional image data in which θ, z are set to two axes, by calculating a representative pixel value P (θ, z) in a viewing direction toward the radial direction from the rotation center of the imaging core in each line data L (θ, z), where the rotation angle of the imaging core is defined as θ and the line data at a movement position z in the movement is defined as L (θ, z); and a display for displaying the calculated two-dimensional image data.