[en] We developed a simple geometric calibration method for modified animal pinhole single photon emission computed tomography (SPECT) imaging systems using a laser alignment system that does not require additional calibration scans, and verified the feasibility of this system. An optical system consisting of a laser generator and a reflecting mirror, and an alignment method using this system were developed. After laser alignment had been completed, SPECT scans of a Tc-99m line source were performed. After acquiring data with complete alignment, the rotation stage was moved along the y-axis (parallel to the detector plane) to acquire off-axis data. Using these on- and off-axis settings, a micro-performance phantom with hot-rod inserts was scanned after filling with Tc-99m solution. Reconstructed images of the line source and the hot-rod phantom showed that image degradation was minimized when the rotation center was completely aligned using our system. The spatial resolution of the reconstructed image measured using the line source was finest under complete alignment. Under this condition, the smallest hot rod (1.2 mm diameter) was resolved in the SPECT image acquired using a 0.5 mm pinhole aperture. The effects of misalignment were clearly observable in off-axis images; only hot rods with more than 3.2 mm diameter were resolved in 1 mm off-axis image whereas there were no resolvable hot rods with more than 2 mm off-axis image. This system of which the feasibility was verified in the present study will be useful for low-cost molecular imaging studies using single photon emitting tracers