In order to solve a problem that a local optical characteristic-changed region inside an object cannot be accurately estimated, an object observing apparatus includes: a light intensity information acquiring unit that acquires light intensity information received by each light-receiving probe; a light intensity change information acquiring unit that acquires, for each probe set, light intensity change information, from reference light intensity information and light intensity information; an estimating unit that acquires three-dimensional optical characteristic-changed region information, using the light intensity change information; and an output unit that outputs the optical characteristic-changed region information; the estimating unit including a correcting part that performs correction according to sensitivity attenuation in accordance with a depth; and a sparseness applying part that introduces sparseness for improving a space resolution, thereby acquiring the optical characteristic-changed region information. Accordingly, it is possible to accurately estimate a local optical characteristic-changed region inside an object.