To obtain a weight sensor manufactured by a low cost by simply altering a weight range to be measured.

A weight sensor 10 comprises a container 12 having flexibility. Fluid 14 such as the air, water or oil is sealed in the container 12. A pressure sensor 16 is mounted in an opening of the container 12. A pressure receiving plate 18 is placed on the container 12, and material 20 to be measured is placed on the plate 18. A weight of the material 20 is transmitted to fluid 14 via the plate 18, and pressure change of the fluid 14 is detected by the sensor 16. A weight of the material 20 is detected from an output signal of the sensor 16 and an area of the plate 18.