A thread gauge, which allows to reliably know the time for replacement by visual checking and improves the predetermined dimensional accuracy of a thread product has a go-side thread measuring portion extending from one side of a shank portion coaxially with the shank portion or a not-go-side thread measuring portion extending from an other side of the shank portion coaxially with the shank portion, a wear checking gauge line having a predetermined length and a predetermined depth formed on male thread portions and female thread portions formed on the go-side thread measuring portion or the not-go-side thread measuring portion.