To provide a quartz sensor of low cost, capable of detecting easily an object material, treating easily especially a contaminated tool after a work, and reducing extremely a waste quantity.

This sensor comprises a quartz piece wherein a flat recessed part for storing the object material to be detected is formed on a center part, an excitation electrode formed in the facing state to the recessed part of the quartz piece, a leading electrode for leading the excitation electrode to the end of the quartz piece, and a reactant to be reacted with the object material to be detected, formed in the recessed part.