A medical device includes a casing, a cover member that covers a portion on one-direction side of the same, and a sheet member interposed therebetween. The cover member includes on its periphery a hook part having a hook-like portion and a protrusion. The casing includes a latch part configured to engage with the hook part and a fitted portion for the protrusion to fit in. The sheet member includes an alignment part for aligning itself with the cover member. The cover member preferably has optical transparency. The casing preferably includes a recess shaped to fit with an outer shape of the cover member at a portion to be covered by the cover member.