An optical transmitter in which a microstripline is formed by a signal wiring pattern and a GND conductor pattern on a circuit board (including a flexible board), and a signal lead of a light emitting element module is mounted in a signal lead mounting hole connected with said signal wiring pattern so as to be connected with said signal wiring pattern, wherein a pattern adapted to add capacitance between a stem of said light emitting element module and said signal wiring pattern is provided on said signal wiring pattern on said circuit board, and wherein the pattern adapted to add capacitance has a size corresponding to .+-.50% of a capacitance C.apprxeq.L/Zo.sup.2, where Zo is signal line impedance, and L is the signal lead inductance.