PURPOSE: To construct a circular channel having excellent jointing ability with base concrete when the circular water channel is constructed through a slip form method and to simply and reliably prevent the occurrence of sagging of the slit part of the constructed annular channel.

CONSTITUTION: By using forms 12 for base concrete, base concrete 2 on the upper surface of which a pair of right and left protrusions 2a in parallel in the longitudinal direction of a channel are formed is continuously placed and shaped through a slip form method. After reinforcing bars 3 are arranged thereon, forms 13 for a circular channel comprising outer forms 13A and inner forms 13B consisting of a circular part 13a and a slit part 13b are used and the circular channel 4 is continuously concreted and shaped on the base concrete 2 through the slip form method. A light cover 5 formed of rigid foamed styrene is fitted in the slip 4b of the circular channel 4 separated away from the forms 13. After concrete is cured, the light cover 5 is removed.