An information processing apparatus that is configured to transmit an outgoing message to another information processing apparatus connected via a network which is a CAN or CAN FD network and is also configured to receive a message transmitted from the other information processing apparatus the information processing apparatus comprising: a shared information generation unit configured to generate shared information to be shared with the other information processing apparatus; a communication unit configured to receive the message and transmit the outgoing message; a shared information verification unit configured to compare shared information included in the message with the shared information generated by the shared information generation unit, and verify validity of the shared information included in the message on the basis of a result of the comparison; a security code generation unit configured to generate a security code based on the shared information; a security code verification unit configured to compare a security code included in the message with the security code generated by the security code generation unit, and verify correctness of the shared information included in the message on the basis of a result of the comparison; and a processing determination control unit configured to determine normality of the message on the basis of a result of the verification by the shared information verification unit and a result of the verification by the security code verification unit; wherein the security code generation unit generates, as the security code, an error detecting code for detecting an error in the shared information, and the security code verification unit compares an error detecting code included in the message with the error detecting code generated by the security code generation unit, and verifies the correctness of the shared information included in the message on the basis of a result of the comparison; or wherein the security code generation unit generates, as the security code, an error correcting code for detecting and correcting an error in the shared information, and the security code verification unit compares an error correcting code included in the message with the error correcting code generated by the security code generation unit, and verifies the correctness of the shared information included in the message on the basis of a result of the comparison; and characterised in that the processing determination control unit determines that the message is normal when the shared information verification unit determines that the shared information included in the message is valid, and determines that the message is due to an illegal attack when the shared information verification unit determines that the shared information included in the message is not valid and the security code verification unit determines that there is no error in the shared information included in the message. a shared information generation unit configured to generate shared information to be shared with the other information processing apparatus; a communication unit configured to receive the message and transmit the outgoing message; a shared information verification unit configured to compare shared information included in the message with the shared information generated by the shared information generation unit, and verify validity of the shared information included in the message on the basis of a result of the comparison; a security code generation unit configured to generate a security code based on the shared information; a security code verification unit configured to compare a security code included in the message with the security code generated by the security code generation unit, and verify correctness of the shared information included in the message on the basis of a result of the comparison; and a processing determination control unit configured to determine normality of the message on the basis of a result of the verification by the shared information verification unit and a result of the verification by the security code verification unit;