To provide an invert construction method for efficiently constructing in a short period an invert constructed by placing concrete on a base layer and formed with a drainage ditch along the center part.

The invert construction method for forming the invert 12 provided with the drainage ditch 14 at the center part, in a pit of a tunnel 10 comprises a base layer forming step of forming the base layer 15 at the bottom part 11 in the pit, a block installation step of fixing a recessed block 16 having an approximately U-shaped sectional form part 17 comprising a lateral pair of sidewall parts 16a, 16b and a bottom wall part 16c, at the center part of the formed base layer 15 continuously in the axial direction of the tunnel 10, a concrete placing step of placing invert concrete 18a, 18b on both sides of the installed recessed block 16, using the recessed block 16 as a form on the drainage ditch 14 side, and a drainage ditch forming step of finishing the inside bottom part 19 of the recessed block 16 to a predetermined depth to form the drainage ditch 14.