ABS system does a self check that is why light goes off like normal, then the self check takes place, the check fails and the light comes on permanently. Usually such problems are triggered by a faulty wheel speed sensor or most probably by a faulty/broken sensor wire. If you want to do the job yourself take the car to a service station, get the car on the lifter and get under the car to look at the ABS sensors located at each of the four wheel hubs. Then trace and follow the ABS wire back to the connectors and look for breaks in the wire. If wire continuity checks out then it requires professional attention. Best place is the Toyota 3S place in your city.