[Task]To provide a sensor fixation device capable of preventing loss of mount nuts.[Solution]A mount body 130 of a sensor fixation device 100 in a mount state includes a first side wall 133a and a second side wall 133b which face with each other at the other end, which are perpendicular to a trap opening axis JT. The first side wall 133a includes a first mount nut retaining opening 133a1, and the second side wall 133b includes a second mount nut retaining opening 133b1. A mount nut 170 includes a nut spherical part 173 interposed between the first side wall 133a and the second side wall 133b, which has a diameter longer than the distance between the first side wall 133a and the second side wall 133b, and is configured to be engaged with a first side wall opening 133a1 and a second side wall opening 133b1. The mount nut 170 is retained integrally with the mount body 130, ensuring to prevent loss of only the mount nut 170 during the mount work.