A child seat attached to a vehicle, comprising: a frame body which is contained in a base supporting a main body seating and restraining a child, which stiffens the base , and which is fixed and disposed in the base so that a front end is exposed from a notch formed in the base ; a rotational support attached to a site exposed at the base front end of the frame body to be rotatable around the frame body , and containing a lock mechanism capable of being locked and unlocked at a predetermined angle with respect to the frame body ; and a support leg including a leg support capable of adjusting and holding a leg portion coupled to the rotational support at a predetermined length, wherein the base is fixed onto a vehicle seat , the support leg is rotated around the rotational support and adjusted to have a predetermined length at a base front end position, and the base is stably held by abutting a bottom plate of the leg support on a vehicle body floor , wherein the frame body comprises a pipe material that is processed substantially into u-shape including a front end frame located on the front end of the base , and a hardened frame connecting both sides of the front end frame and extending in a longitudinal direction of the base , and the rotational support is rotatably attached to the front end frame , characterized in that the lock mechanism locks by inserting a lock pin biased by a biaser contained in inner portion into a through-hole formed in the frame body , and unlocks by pulling out the leg portion of the support leg from the rotational support by a predetermined distance against the biaser. a frame body which is contained in a base supporting a main body seating and restraining a child, which stiffens the base , and which is fixed and disposed in the base so that a front end is exposed from a notch formed in the base ; a rotational support attached to a site exposed at the base front end of the frame body to be rotatable around the frame body , and containing a lock mechanism capable of being locked and unlocked at a predetermined angle with respect to the frame body ; and a support leg including a leg support capable of adjusting and holding a leg portion coupled to the rotational support at a predetermined length, wherein the base is fixed onto a vehicle seat , the support leg is rotated around the rotational support and adjusted to have a predetermined length at a base front end position, and the base is stably held by abutting a bottom plate of the leg support on a vehicle body floor , wherein the frame body comprises a pipe material that is processed substantially into u-shape including a front end frame located on the front end of the base , and a hardened frame connecting both sides of the front end frame and extending in a longitudinal direction of the base , and the rotational support is rotatably attached to the front end frame , characterized in that the lock mechanism locks by inserting a lock pin biased by a biaser contained in inner portion into a through-hole formed in the frame body , and unlocks by pulling out the leg portion of the support leg from the rotational support by a predetermined distance against the biaser. <EOS>