Put

プット・オプション。原資産100株を権利行使価格で売る権利を保有者に与えるオプションで、株価が下落すると価値が増す。

A put is a contract that gives you the right, but not the obligation, to sell 100 shares of a stock at a fixed strike price before the option expires. Its value rises when the underlying falls, so traders reach for puts when they expect a drop or want to protect a position they already own. Selling a put is the opposite bet: you collect the premium and take on the obligation to buy the shares if they fall below the strike.

Say a stock trades at 50 and you buy a 45 put for 1.20. If the stock slides to 40 before expiry, that put is worth at least 5, since you can sell at 45 what the market values at 40. If the stock stays above 45, the put expires worthless and your 1.20 is gone. That premium is the full cost of the insurance, whether or not you ever use it.

A common mistake is treating a long put as a guaranteed win on any dip. Time decay works against you every day, and a small move that arrives too slowly can still leave you with a loss. If you only want downside protection rather than a directional trade, size the strike and expiry around the position you are actually hedging.

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