A laser diode driving device (1) of the present invention includes: a variable DC power supply (3) for outputting a voltage for driving laser diodes (LD1 to LDn); a current driving element (4) for causing a current If to flow; a current control section (5) for controlling (i) turning on and off of the current driving element (4) and (ii) the amount of the current If; and a power supply control section (7) for controlling an output voltage of the variable DC power supply (3). The power supply control section (7) controls, on the basis of If-Vf characteristic data (D2), voltage drop characteristic data (D3), and Vds setting data (D4) all stored in a memory section (8), the output voltage of the variable DC power supply (3) so that the current driving element (4) has a constant inter-terminal voltage regardless of the amount of the current If.