A controller for controlling interruptions and restarts when writing data to a recording medium by emitting a laser beam against the recording medium. The controller prevents buffer underrun errors. The laser beam is emitted at a high power level when writing data on the recording medium and at a low power read level when reading data from the medium. The writing of data is interrupted when there is a possibility of a buffer underrun error. When restarting data writing from where the interruption occurred, the laser beam is shifted from the read level to the write level before reaching the location at which the interruption occurred. This guarantees that the laser beam has the required power level when writing is restarted.