A microfluidic device includes a resistor. In focus pull-in performed after a learning process, a reference-position control unit positions the objective lens at a reference position obtained through a learning process in accordance with the reference current, and then, causes the focus pull-in control unit to perform focus pull-in. The power line transceiver operates in a transmitter mode as an intentional radiator to emit modulated carrier signals by transmitting them at a frequency band and a power level sufficient to form by electrical conduction a communication link with a modulated carrier signal receiving device. A reduction of the diameter of the retainer sleeve is prevented by the presence of the ring member. The search start point is identified based on the reference period and a predetermined first correction time.