The water pressure, varied as the water level changes, is measured with a quartz oscillator, whose oscillating frequency is converted to optical signals (SM) at the E/O converter inside the sensor for transmission. The power source of the sensor receives the laser light, emitted from the light source built in the water level gauge coder, at the optical electric power supply to generate the electric power. Free from lightning surge as a wireless cable (optical fiber) is used for data transmission between the sensor and the signal converter. High accuracy measurement, without a conversion error caused by the secular change commonly seen in diaphragm A wide measurement range, from 10m to 70m, applicable not only to rivers but also to dams.