A signal processing circuit substrate is formed with a through-hole. A device having a variable value and including an value adjustment portion through which the variable value is adjusted is mounted on a first surface of the signal processing circuit substrate. The signal processing circuit substrate includes a flexible arch-shaped member having a height relative to the first surface of the signal processing circuit substrate. The device is electrically and mechanically fixed onto a lower surface of the member in a floating condition above the signal processing circuit substrate such that the value adjustment portion is in alignment with the through-hole so as to allow the value adjustment portion to be adjusted through the through-hole. The member is fixed at opposite edges onto the first surface of the signal processing circuit substrate.