When we use computed tomography in clinical cases, it is important to know the slice thickness of the body displayed by computed tomography. We are informed that the unit is equiped with A and B slice with 10 mm thickness. However, it is necessary to know what layer is scanned by computed tomography. For convenience "sake" we made a computed tomographic test phantom. It was acrylate plate of 10 mm thickness. The holes, 2 mm in diameter, were made with a distance of 2.5 mm between each hole. The total thickness of this test phantom was 60 mm in piles. This test phatom was placed in the center of rotation, measuing the slice thickness. The slice thickness can be obtained by counting the number of holes in the computed tomographic image. The present results showed that the slice thickness of A was 7.5 mm for low absorption objects, and 10 mm for high absorption objects. The thickness of B slice was 10 mm for the former and 12.5 mm for the latter, respectively.