Study on high speed indoor wireless optical LAN system enabling 100Mbps signal transmission with low bit error rate (10-9) is presented. To realize the optical LAN system handling 100 Mbps signal, a directed line of sight (LOS) system is adopted as the optical receiver sensitivity for a bit error rate of 10-9 for 100 Mbps signals is fairly large. In the system, new approaches are introduced: WDM technology which enables bi-directional transmission in full duplex manner is applied using a 1.3 micrometers laser diode for down-link and 0.65 micrometers red laser diode for up-link light sources. As the wavelengths of the two lasers are quite separated from each other, this WDM technology brings an advantage that two kind of semiconductor materials can be used for detectors; GaInAs is used for down-link while Si is applied for up-link. GaInAs PD cannot detect the up-link laser light of 0.65 micrometers and Si PD or APD cannot detect the down-link laser light of 1.3micrometers . Therefore full duplex transmission can be achieved in this configuration. In the indoor wireless optical LAN system, one of the critical points is the transmitter configuration for down- link which enables to deliver optical power enough for 100 Mbps transmission to user areas as wide as possible with inexpensive prices. To realize the point, a special 1.3micrometers laser diode, a spot-size converter integrated laser (SS-LD), is introduced in company with convex lens and an object lens to deliver optical power to areas as wide as possible. As the far-field patterns of the SS-LD are fairly narrow, most of the output power of the LD could be collected to and spread wide by the object lens of 40 magnifications. Using the device, 3m diameter circle area in the plane 2m apart from the 1.3micrometers SS-LD emitting 20 mW optical power, could receive optical power above the receiver sensitivity for a bit error rate of 10-9 for 100 Mbps signals. The visible red light is convenient for not only position setting but also eye-safety problem. Although maximum permissible exposure value at 0.65micrometers is lower than those at longer wavelength, winking response of human eyes for the visible laser light results in higher exposure value. Using 0.65 micrometers visible laser light of 1 mW output power for up- link light source and Si APD for detectors, reasonable area which can receive optical power above the receiver sensitivity for a bit error rate of 10(superscript -9 for 100 Mbps signals was obtained.