A tire testing apparatus of drum type, comprising: a drum having, on a circumferential outer surface thereof, a sensor for measuring a ground contact force of a tire and a transparent portion; an image capturing device for capturing, from inside of the drum and through the transparent portion, an image of the ground contact surface of the tire; a ground contact force measurement processing unit for measuring a ground contact force of the tire in planar contact with the sensor; a ground contact surface image capture processing unit for capturing an image of the ground contact surface by the image capturing device when the tire comes into planar contact with the transparent portion; and a slip amount calculation processing unit which is adapted to calculate a slip amount based on the displacement of a corresponding location in images by using an image of the ground contact surface of whole or part of the circumference of the tire under a first load and at a first speed in a preprocessing execution, wherein first load and first speed are chosen such as to cause no slip in the ground contact surface of the tire, and the image of the ground contact surface of whole or part of the circumference of the tire under a second load and at a second speed in the actual processing execution, wherein the tire testing apparatus: is adapted to detect an angle θ T of a tire shaft that supports the tire and an angle θ D of a drum shaft that supports the drum; and is adapted to control the rotation of the tire and/or the drum so that there occurs a timing when the relationships θ T =θ 0 and θ D =θ 1 are satisfied and a timing when the relationships θ T =θ 0 and θ p =θ 2 are satisfied, where θ 0 is an angle from a reference point of the tire shaft when the tire is in contact with the drum, θ 1 is an angle from a reference point of the drum shaft when the image capturing device becomes capable of capturing the ground contact surface of the tire through the transparent portion, and θ 2 is an angle from a reference point of the drum shaft when the ground contact surface of the tire comes into contact with the sensor. a drum having, on a circumferential outer surface thereof, a sensor for measuring a ground contact force of a tire and a transparent portion; an image capturing device for capturing, from inside of the drum and through the transparent portion, an image of the ground contact surface of the tire; a ground contact force measurement processing unit for measuring a ground contact force of the tire in planar contact with the sensor; a ground contact surface image capture processing unit for capturing an image of the ground contact surface by the image capturing device when the tire comes into planar contact with the transparent portion; and a slip amount calculation processing unit which is adapted to calculate a slip amount based on the displacement of a corresponding location in images by using an image of the ground contact surface of whole or part of the circumference of the tire under a first load and at a first speed in a preprocessing execution, wherein first load and first speed are chosen such as to cause no slip in the ground contact surface of the tire, and the image of the ground contact surface of whole or part of the circumference of the tire under a second load and at a second speed in the actual processing execution, wherein the tire testing apparatus: is adapted to detect an angle θ T of a tire shaft that supports the tire and an angle θ D of a drum shaft that supports the drum; and is adapted to control the rotation of the tire and/or the drum so that there occurs a timing when the relationships θ T =θ 0 and θ D =θ 1 are satisfied and a timing when the relationships θ T =θ 0 and θ p =θ 2 are satisfied, where θ 0 is an angle from a reference point of the tire shaft when the tire is in contact with the drum, θ 1 is an angle from a reference point of the drum shaft when the image capturing device becomes capable of capturing the ground contact surface of the tire through the transparent portion, and θ 2 is an angle from a reference point of the drum shaft when the ground contact surface of the tire comes into contact with the sensor. is adapted to detect an angle θ T of a tire shaft that supports the tire and an angle θ D of a drum shaft that supports the drum; and is adapted to control the rotation of the tire and/or the drum so that there occurs a timing when the relationships θ T =θ 0 and θ D =θ 1 are satisfied and a timing when the relationships θ T =θ 0 and θ p =θ 2 are satisfied, where θ 0 is an angle from a reference point of the tire shaft when the tire is in contact with the drum, θ 1 is an angle from a reference point of the drum shaft when the image capturing device becomes capable of capturing the ground contact surface of the tire through the transparent portion, and θ 2 is an angle from a reference point of the drum shaft when the ground contact surface of the tire comes into contact with the sensor.