1. A user terminal apparatus comprising: an input sensing unit configured to sense a user's input to the user terminal apparatus; an information collecting unit configured to collect information regarding input states, which are determined based on the user's input, and a transition of the determined input states; and a security-control unit configured to determine whether the user is valid based on at least a comparison of the collected information with contextual information, and controls a security function, wherein the transition of the determined state includes the staying in the one input state and the changing from one input state to another input state, wherein the security-control unit compares a number of times that a specific input state occurs among the determined input states with a predetermined reference number of times to determine whether the user is valid, and wherein when the information is collected a sample number of times, which is a multiple of the predetermined reference number of times that the information to determine whether the user is valid is collected and is obtained by counting only the cases in which the user is determined to be valid, the security-control unit updates previously set probability information, which includes probabilities of occurrence of each of the input states and probabilities of transitions between the input states, using probability information that is obtained based on information collected only when the user is determined to be valid, from among pieces of information which are collected until the sample number of times is reached, and resets the predetermined number of times that the information to determine whether the user is valid is collected and the predetermined reference number of times based on the updated probability information.