To make determinable the permittivity of a dielectric by projecting electromagnetic wave to the dielectric, receiving reflection wave for a buried reflector, image-processing obtained two-dimensional plane data and changing parameters corresponding to wave transmission velocities.

The permittivity measuring device 1 is put on the ground or concrete surface being a dielectric and a microwave is emitted from a sensor part 2 toward underground. The reflection wave of an object 6 buried underground is operated for processing with an operation processor 3 and two-dimensional display image data and three-dimensional display image data sliced in the depth direction are indicated on a display 4. Here, the wavelength transmission velocity ν of radar wave as a factor to obtain the burying depth Z0 of the object 6 of an underground radar is obtained from ν=2×Z0/t (t: return time of reflection wave of the object 6) with the operation processor 3. This wavelength transmission velocity ν is changed as parameter and a wavelength transmission velocity ν where an image is most clearly obtained is selected. The permittivity r is obtained by ν=c/√ r (c: light velocity) in the operation processor 3.