A robot system (120; 120A) comprising: a robot (1; 1A; 1B); a portable operation terminal having a teaching function; and a robot controller (7; 7A) having operation modes including a teaching mode and another operation mode different from the teaching mode, the teaching mode being an operation mode for teaching a motion to the robot (1; 1A; 1B) through the portable operation terminal, wherein the portable operation terminal is attachable to and detachable from the robot controller (7; 7A); and wherein the robot controller (7; 7A) is configured to: detect electrical connection and electrical disconnection between the robot controller (7; 7A) and the portable operation terminal; change an operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode when detecting the connection between the robot controller (7; 7A) and the portable operation terminal; and change the operation mode in which to operate the robot (1; 1A; 1B) to the other operation mode different from the teaching mode when detecting the disconnection between the robot controller (7; 7A) and the portable operation terminal, the robot system being characterised in that the robot controller is configured to: detect the disconnection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is detached from the robot controller (7; 7A), with no other mode change operation being performed, and detect the connection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is attached to the robot controller (7; 7A), with no other mode change operation being performed, and upon detecting the connection, automatically change the operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode. a robot (1; 1A; 1B); a portable operation terminal having a teaching function; and a robot controller (7; 7A) having operation modes including a teaching mode and another operation mode different from the teaching mode, the teaching mode being an operation mode for teaching a motion to the robot (1; 1A; 1B) through the portable operation terminal, wherein the portable operation terminal is attachable to and detachable from the robot controller (7; 7A); and wherein the robot controller (7; 7A) is configured to: detect electrical connection and electrical disconnection between the robot controller (7; 7A) and the portable operation terminal; change an operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode when detecting the connection between the robot controller (7; 7A) and the portable operation terminal; and change the operation mode in which to operate the robot (1; 1A; 1B) to the other operation mode different from the teaching mode when detecting the disconnection between the robot controller (7; 7A) and the portable operation terminal, the robot system being characterised in that the robot controller is configured to: detect the disconnection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is detached from the robot controller (7; 7A), with no other mode change operation being performed, and detect the connection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is attached to the robot controller (7; 7A), with no other mode change operation being performed, and upon detecting the connection, automatically change the operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode. detect electrical connection and electrical disconnection between the robot controller (7; 7A) and the portable operation terminal; change an operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode when detecting the connection between the robot controller (7; 7A) and the portable operation terminal; and change the operation mode in which to operate the robot (1; 1A; 1B) to the other operation mode different from the teaching mode when detecting the disconnection between the robot controller (7; 7A) and the portable operation terminal, the robot system being characterised in that the robot controller is configured to: detect the disconnection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is detached from the robot controller (7; 7A), with no other mode change operation being performed, and detect the connection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is attached to the robot controller (7; 7A), with no other mode change operation being performed, and upon detecting the connection, automatically change the operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode. detect the disconnection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is detached from the robot controller (7; 7A), with no other mode change operation being performed, and detect the connection between the robot controller (7; 7A) and the portable operation terminal when the portable operation terminal is attached to the robot controller (7; 7A), with no other mode change operation being performed, and upon detecting the connection, automatically change the operation mode in which to operate the robot (1; 1A; 1B) to the teaching mode.