A device for measuring mechanical quantity is provided which reduces the influence of a difference in thermal expansion coefficient between an object to be measured and a base plate metal body, and precisely measures a mechanical quantity such as deformation quantity or strain quantity caused in the object to be measured. The device includes a semiconductor strain sensor module for measuring deformation quantity of the object to be measured, and the module includes a metal body, and a semiconductor strain sensor mounted on the metal body to detect strain of the metal body. The object to be measured is made of a material having a thermal expansion coefficient larger than that of the metal body. Further, the metal body mounted with the semiconductor strain sensor has a structure configured to be fixed to the object to be measured.