I have not lost the screw for mine, but the screw looks like it is a #4 x 1/2". Maybe a #4 x 3/8". A good hardware store should have it. Also a repair kit for glasses should have an assortment of tiny screws. I would think that one of those should fit.
I also lost my screw and contacted Omron. They are unable to send me a screw but I can mail my pedometer back to Omron and they will diagnose the problem, repair, and mail it back to me if I enclose $5 for return postage. This of course is if the warranty is in effect, otherwise I guess they charge to replace the screw. I think I'll use tape or try to find a small screw that fits. Am I missing a liability issue in mailing someone a small screw?
Just got one of these pedometers for my wife. She dropped it while installing the battery and lost the screw. I just got off the phone with Omron, they won't send me a screw. As stated above, Omron says I have to ship it to them and enclose $8 to cover having it shipped back to me. Seems ridiculous as this will almost equal what I paid for the thing to begin with. Hopefully I can find a better solution. Currently, she is holding the battery cover in place with the belt clip but occasionally it seems the battery loses contact because the count restarts.
This tiny battery cover screw is a rediculous design flaw in an otherwise good instrument. Surely, they could have done something better - if only including scew type/number in Manuel. I have been on my hands and knees for 30 minutes looking for darn thing!