Membrane switch, keyboard, and electronic apparatus having keyboard

According to one embodiment, a membrane switch includes an insulating sheet provided with a first sheet portion having a first contact, a second sheet portion having a second contact, and a fold-back portion having a fold line. The fold-back portion has a conductor pattern which crosses the fold line. A gap in which the conductor pattern is to be situated is defined inside the fold-back portion by folding the fold-back portion of the insulating sheet along the fold line so that the first contact and the second contact face each other. The gap is filled with an adhesive having electrical insulating properties and water-repellent properties. The adhesive covers the conductor pattern.