Abstract: A chemical sensing element suitable for measurement of the activity or the concentration of a specific ion contained in an electrolyte is provided with an optically opaque membrane between a gate insulating membrane and an ion sensing membrane of a field-effect type transistor. Since the opaque membrane is provided, extraneous light impinging on a gate is blocked by the opaque membrane, whereby an erroneous potential due to extraneous light is prevented. Accordingly, the accurate measurement of the activity or the concentration of a specific ion contained in an electrolyte may be achieved even in intense ambient light.