A communication adapter includes: an input unit receiving an operation for requesting transition to a setting mode; a mode setting unit setting a communication mode to the setting mode when the input unit receives the operation for requesting transition to the setting mode; a character string generation unit generating a random character string when receiving a connection start request from a terminal in the setting mode; an image generation unit converting the random character string into image data indicating an image that is difficult for a machine to recognize; an encryption processing unit encrypting transmission data to be transmitted to the terminal using the random character string as an encryption key, and decrypting reception data received from the terminal using the random character string; and a communication processing unit transmitting the image data and the encrypted transmission data to the terminal, and receiving the reception data from the terminal.