A state detecting device which can be applied even in a severe environment. The state detecting device includes a chargeable all-solid-state battery, a piezoelectric element which supplies charging power to the all-solid-state battery, and an integrated circuit including various sensors which operate with electric power supplied from the all-solid-state battery. The all-solid-state battery, the piezoelectric element, and the integrated circuit are mounted on one surface of a flexible substrate. The flexible substrate is attached to a flexible object which is either an object to be measured or constitutes at least part of an inner surface of a space to be measured.