In a first aspect of the invention an adjustable hairbrush includes an inner cylinder to which are affixed brush bristles and an outer cylinder through which the brush bristles extend. A method of manufacturing flexible pipe body is also disclosed. A shift in threshold voltage of a transistor which easily deteriorates and a shift in threshold voltage of a turned-on transistor are suppressed by signal input to a gate electrode of the transistor which easily deteriorates through the turned-on transistor.