A touch panel apparatus comprising: a first substrate in which a first conductive film is formed thereon; a second substrate in which a second conductive film is formed thereon; a first terminal, a second terminal, a third terminal, and a fourth terminal respectively provided on four corners of the second substrate; and a controller, wherein the controller is configured to: measure a first potential of the first terminal and a second potential of the second terminal, in a state where the first terminal and the second terminal are connected to the power source potential and the third and fourth terminals (33, 34) are connected to a ground potential, to obtain a first potential difference between the first potential and a first reference potential and a second potential difference between the second potential and a second reference potential, measure a third potential of the second terminal and a fourth potential of the third terminal in a state where the second terminal and the third terminal are connected to the power source potential and the first and fourth terminals (31, 34) are connected to the ground potential, to obtain a third potential difference between the third potential and a third reference potential and a fourth potential difference between the fourth potential and a fourth reference potential, select one of the potential differences that has the largest value among the first potential difference, the second potential difference, the third potential difference and the fourth potential difference, and detect a positional relationship between two contact points based on the selected one of the potential differences. a first substrate in which a first conductive film is formed thereon; a second substrate in which a second conductive film is formed thereon; a first terminal, a second terminal, a third terminal, and a fourth terminal respectively provided on four corners of the second substrate; and a controller, wherein the controller is configured to: measure a first potential of the first terminal and a second potential of the second terminal, in a state where the first terminal and the second terminal are connected to the power source potential and the third and fourth terminals (33, 34) are connected to a ground potential, to obtain a first potential difference between the first potential and a first reference potential and a second potential difference between the second potential and a second reference potential, measure a third potential of the second terminal and a fourth potential of the third terminal in a state where the second terminal and the third terminal are connected to the power source potential and the first and fourth terminals (31, 34) are connected to the ground potential, to obtain a third potential difference between the third potential and a third reference potential and a fourth potential difference between the fourth potential and a fourth reference potential, select one of the potential differences that has the largest value among the first potential difference, the second potential difference, the third potential difference and the fourth potential difference, and detect a positional relationship between two contact points based on the selected one of the potential differences. measure a first potential of the first terminal and a second potential of the second terminal, in a state where the first terminal and the second terminal are connected to the power source potential and the third and fourth terminals (33, 34) are connected to a ground potential, to obtain a first potential difference between the first potential and a first reference potential and a second potential difference between the second potential and a second reference potential, measure a third potential of the second terminal and a fourth potential of the third terminal in a state where the second terminal and the third terminal are connected to the power source potential and the first and fourth terminals (31, 34) are connected to the ground potential, to obtain a third potential difference between the third potential and a third reference potential and a fourth potential difference between the fourth potential and a fourth reference potential, select one of the potential differences that has the largest value among the first potential difference, the second potential difference, the third potential difference and the fourth potential difference, and detect a positional relationship between two contact points based on the selected one of the potential differences.