Satisfactory observation will be enabled by extending/dilating an organ such as the stomach to be observed to secure a sufficient field of view. A capsule endoscope (4) can be caused to float at an interfacial boundary (12) between a first liquid (7) and a second liquid (8) with more stability than at an interfacial boundary between one liquid and air by inserting, together with the first liquid (7) and the second liquid (8) whose specific gravity is smaller than that of the first liquid (7) and which does not mix with the first liquid, the capsule endoscope (4) having an intermediate specific gravity between the specific gravity of the first liquid (7) and that of the second liquid 8 into a stomach (3) of a subject (2) and also, satisfactory observation is enabled by securing a sufficient field of view inside the desired stomach (3) extended/dilated sufficiently over an upper area from the interfacial boundary (12) after the second liquid (8) being inserted, in addition to the first liquid (7).
