Congratulations, Shan. You have a valuable coin. It must, of course, be dated 1903 and it must be genuine to be valuable. The date is relatively easy to determine (see below). Authenticity, however, is sometimes not that easy.
Sad to say there is a new crop of coin counterfeiters who have been coming to numismatics (coin collecting) over the past decade or two. They are equipped with high technology and well-backed by crooks. You should never buy a valuable coin unless you know for sure that it is genuine. The only sure-fire method is the deal only with well-established dealers with whom you have a long relationship.
More often than not, precise weight indicates authenticity. A genuine 5 sen weighs 4.67 grams. If Shan's coin weighs precisely this much, it is very likely a genuine coin. If not, it is a fake. These coins are made of copper-nickel.
Even though Shan has the *key date* of 1903, there are several other dates in the series that carry catalog values something like this.
JAPAN 5 SEN EXCEPT 1903:
worn; $5 US dollars approximate catalog value
average circulated (like our picture): $10
well preserved: $35
fully uncirculated: $140
As always on CoinQuest, you must apply the concepts presented on our Terminology page to convert these inflated catalog values to actual buy and sell values.
As to the value of the key date 1903, the first order of business is the verify the correct date. AllCalendars.net has a good explanation of Japanese dates and a converter that lets you enter a Western date and obtain a Japanese one. The picture to the left shows a coin dated 1903.
Here are approximate catalog values for 1903-dated 5 sen:
JAPAN 5 SEN DATED 1903:
worn; $120 US dollars approximate catalog value
average circulated (like our picture): $220
well preserved: $350
fully uncirculated: $2200