Seat belt guiding auxiliary device and juvenile car seat

A seat mat (1) comprises a seat base (20) set on a bench sheet of a car and laid under buttocks of a sitting person, a pair of loop parts (30, 40) extending upward from forward both side edges of the seat base (20) along both sides of the buttocks of the sitting person and passing a waist belt part of a car seat belt, and a pair of slanted connecting members (50) connecting the upper ends of the loop parts (30, 40) to backward both side edges of the seat base (20).