To provide a construction method for a groove section in a concrete floor surface capable of easily and surely carrying out work when a floor surface or the like of a veranda for a concrete building such as an apartment block or the like, a drain gutter or the like in the concrete floor surface are to be constructed.

An adjusting device (A) used for the construction of the groove section in the concrete floor surface includes a mounting member (1) fixing a groove forming block (B) forming the groove section, the front section supporting device (2) capable of adjusting the height by supporting a handrail construction section (93) of the mounting member (1), the back section supporting device (3) capable of adjusting the height by supporting the opposite side to the handrail construction section (93), a connecting member (5) connecting the mounting member (1) and a passage adjusting device (4), and the passage adjusting device (4) moving the mounting member (1) back and forth through the connecting member (5), and the front supporting device (2) and the passage adjusting device (4) include a seat members (20) and (40)fixed to a floor slab (90).