A photoacoustic imaging apparatus comprising: a light source; a detector configured to detect an acoustic wave generated from an optical absorber (110, 210, 310) that has absorbed energy of light emitted from the light source, wherein the optical absorber has an optical absorption coefficient being lower or higher than an optical absorption coefficient of a medium surrounding the optical absorber; and a signal processing unit configured to form a signal of an image of the optical absorber, characterized in that the signal processing unit is configured to form the signal of the image of the optical absorber after performing an envelope detection process on the acoustic wave; and the signal processing unit - stores information indicating change in pressure at a start of the acoustic wave generated from the optical absorber detected by the detector before performing the envelope detection process on the acoustic wave, - subsequently to performing the envelope detection process, determines whether the rate of change in pressure at the start of the detected acoustic wave is positive or negative, and - forms the signal of the image such that the image, when displayed, comprises information indicating that the rate of change in the pressure is negative if it is determined that the rate of change in the pressure is negative. a light source; a detector configured to detect an acoustic wave generated from an optical absorber (110, 210, 310) that has absorbed energy of light emitted from the light source, wherein the optical absorber has an optical absorption coefficient being lower or higher than an optical absorption coefficient of a medium surrounding the optical absorber; and a signal processing unit configured to form a signal of an image of the optical absorber, characterized in that the signal processing unit is configured to form the signal of the image of the optical absorber after performing an envelope detection process on the acoustic wave; and the signal processing unit - stores information indicating change in pressure at a start of the acoustic wave generated from the optical absorber detected by the detector before performing the envelope detection process on the acoustic wave, - subsequently to performing the envelope detection process, determines whether the rate of change in pressure at the start of the detected acoustic wave is positive or negative, and - forms the signal of the image such that the image, when displayed, comprises information indicating that the rate of change in the pressure is negative if it is determined that the rate of change in the pressure is negative. - stores information indicating change in pressure at a start of the acoustic wave generated from the optical absorber detected by the detector before performing the envelope detection process on the acoustic wave, - subsequently to performing the envelope detection process, determines whether the rate of change in pressure at the start of the detected acoustic wave is positive or negative, and - forms the signal of the image such that the image, when displayed, comprises information indicating that the rate of change in the pressure is negative if it is determined that the rate of change in the pressure is negative.