A capsule endoscope includes an image sensor that is fixed to a substrate and is covered by a transparent cover member such that a sealed air space is provided between the objective optical system and the image sensor. Also, a capsule endoscope is disclosed that includes a transparent cover member, an image sensor having an imaging area that is covered by the transparent cover member; and an objective optical system which includes at least two lenses. The lens of the objective optical system that is nearest the image side is either integral with, or is adhered to, the transparent cover member. The transparent cover member seals an air space above the imaging area of the image sensor from dust which may be generated during a focusing adjustment of the two lenses. A focusing method adjusts the position of a first lens, in order from the object side, relative to that of a second lens which is nearest the image side.