To provide a device which can easily mark off, in a short period of time, a line parallel to an open end face over the whole circumference of a vessel or a concave part thereof at a position spaced by a predetermined distance from the open end face of the inner surface of the vessel or the concave part.

The device is a marking-off device 1 for marking off a line 44 parallel to an open end face 43 at a position spaced by a predetermined distance from the open end face 43 of an inner surface 42 of a vessel 40 or a concave part thereof having an opened upper surface. The marking-off device 1 comprises a first support 2 supported by the open end face 43 of the vessel 40 or the concave part and a second support 11 that is provided at the first support 2 so as to extend in a bottom direction of the vessel 40 or the concave part, and has a holding part 17, at the front end, which can hold a marking-off tool 35. When the first support 2 is moved along the open end face 43, the marking-off tool 35 is moved along the inner surface 42 of the vessel 40 or the concave part while following the first support 2, whereby the line 44 can be marked off at a predetermined position of the inner surface 42.