In a hot isostatic pressing (HIP) method, only the probe of a dilatometer is set in the pressurized heating space of the HIP apparatus, and the probe is attached to a test piece having a greater specific surface area made of the same material as the body to be treated. The test piece is treated by HIP together with the body to be treated, and the beginning of the contraction of the test piece is detected by the dilatometer. Then, the body is densified by keeping the pressure and the temperature not lower than those at the beginning of contraction of the test piece. According to the method of the invention, since suitable HIP treating conditions are determined immediately, each body to be treated can be treated by only one HIP suitably without repeating the troublesome HIP process. Also, since the body can be treated by HIP without elevating the temperature beyond the necessary temperature, the crystal grain growth of the body to be treated can be inhibited.