A line switching method and apparatus for switching multiplex transmission lines to select, for each of channels of transmission data transmitted in the lines, one of the lines in accordance with line alarm information in the transmission data of the channel, have a pointer value detection function, an alarm detection supervision function and a line switching function for time division multiplexed data of digital signals which are processed serially. The line switching apparatus includes a serial processing circuit provided for each line for detecting line alarm information of the channels by serial processing, and a line switching circuit for switching, for the channels, the lines in a time series relationship in response to the results of detection by the serial processing circuits. Each serial processing section includes a pointer frame top position detection circuit for detecting frame top positions of the channels in multiplexed transmission data from pointer information by serial processing, and a line alarm detection circuit for detecting line alarm information of the channels by serial processing in accordance with the results of detection by the pointer frame top position detection circuit.