1. A proximity determination method that is executed by a computer, the proximity determination method comprising: collecting a first concealed vector based on a linear combination of a row vector of a key matrix, first numerical information, and a first random number, the key matrix being generated by adding a random number vector as a last column to a matrix in which a threshold for determining a proximity range is a diagonal element; collecting a second concealed vector based on the linear combination, second numerical information, and a second random number; collecting a third concealed vector based on another linear combination that is different from the linear combination, the second numerical information, and the second random number; calculating a remainder vector, which is a remainder using the key matrix as a modulus based on a difference between the collected first concealed vector and the second concealed vector; and determining, by the computer, whether the first numerical information and the second numerical information belong to the proximity range, based on the calculated remainder vector.