A coupling comprising: a pair of connection members individually provided on each of a side of a driving shaft and a side of a driven shaft , both the driving shaft and the driven shaft being hollow; a torque transmission portion placed between the pair of connection members , absorbing misalignment, and transmitting torque from the driving shaft to the driven shaft ; a communication passage letting hollow regions of the driving shaft and the driven shaft communicate with each other to form a circulation space (F) for a cooling fluid; and a seal portion sealing between the circulation space and an outside space, wherein the torque transmission portion includes a center tube , and a pair of diaphragms each having an inner diameter portion integrally fixed to both ends of the center tube and an outer diameter portion tightened and fixed to each of the pair of connection members and provided with a deflection portion between the inner diameter portion and the outer diameter portion , and the communication passage is formed by an inner hole of the center tube and an opening provided in each of the diaphragms, the coupling being characterized in that the seal portion includes an endless groove provided in each of joint surfaces (JS1) between the outer diameter portions of the pair of diaphragms and the pair of connection members and an O-ring housed in each of the grooves , at least one of sets of the driving shaft and the driven shaft and the pair of connection members forming a set with the driving shaft and the driven shaft is integrated by surface joining between separate components, and the seal portion includes an endless groove provided in each of joint surfaces (JS2) of two members of a set integrated by the surface joining between the separate components and an O-ring housed in each of the grooves. a pair of connection members individually provided on each of a side of a driving shaft and a side of a driven shaft , both the driving shaft and the driven shaft being hollow; a torque transmission portion placed between the pair of connection members , absorbing misalignment, and transmitting torque from the driving shaft to the driven shaft ; a communication passage letting hollow regions of the driving shaft and the driven shaft communicate with each other to form a circulation space (F) for a cooling fluid; and a seal portion sealing between the circulation space and an outside space, wherein the torque transmission portion includes a center tube , and a pair of diaphragms each having an inner diameter portion integrally fixed to both ends of the center tube and an outer diameter portion tightened and fixed to each of the pair of connection members and provided with a deflection portion between the inner diameter portion and the outer diameter portion , and a center tube , and a pair of diaphragms each having an inner diameter portion integrally fixed to both ends of the center tube and an outer diameter portion tightened and fixed to each of the pair of connection members and provided with a deflection portion between the inner diameter portion and the outer diameter portion , and the communication passage is formed by an inner hole of the center tube and an opening provided in each of the diaphragms, the coupling being characterized in that the seal portion includes an endless groove provided in each of joint surfaces (JS1) between the outer diameter portions of the pair of diaphragms and the pair of connection members and an O-ring housed in each of the grooves , at least one of sets of the driving shaft and the driven shaft and the pair of connection members forming a set with the driving shaft and the driven shaft is integrated by surface joining between separate components, and the seal portion includes an endless groove provided in each of joint surfaces (JS2) of two members of a set integrated by the surface joining between the separate components and an O-ring housed in each of the grooves. <EOS>